[RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Mikko Rantalainen]

This is an RFC because it deliberately changes a long-established raw
syscall ABI. Jan Kara raised userspace-regression concerns when this was
discussed in 2025:

    https://lore.kernel.org/linux-fsdevel/ddqmhjc2rpzk2jjvunbt3l3eukcn4xzkocqzdg3j4msihdhzko@fizekvxndg2d/

while musl and Android bionic already normalize this result to success
in libc.

The Linux implementation of close() will always close the file descriptor
given as argument, except for the invalid file descriptor which will
return -EBADF.

However, currently Linux kernel will return EINTR in some cases for
close(). There is no good way for caller to recover from this case using
the original fd. Whatever action was actually interrupted cannot be
resumed or retried through this fd, because the fd has already been
consumed. Even worse, EINTR conventionally invites retrying an operation,
but retrying close() is unsafe: the same file descriptor number may
already refer to another file opened by another thread by the time
close() returns EINTR.

In addition, POSIX.1-2024 requires that if close() reports EINTR, the
descriptor must remain open. It also explicitly permits an interrupted
close() to return success after closing the descriptor.

This patch is about implementing the second option to be compatible with
both POSIX.1-2024 and real-world applications.

Since close() on Linux first relinquishes the file descriptor and only
then performs ->flush() work, interruption of that later work cannot be
recovered through the original fd. No matter how important that
close-time work was, ownership of the fd has already been irrevocably
relinquished.

For regular files, applications requiring durability already need an
explicit synchronization operation such as fsync() or fdatasync() before
relinquishing the fd. This proposal does not suppress meaningful
delayed-I/O errors such as EIO, ENOSPC, or EDQUOT; it only changes
interruption results whose conventional recovery action (retrying the
operation) is unsafe for close().

Historical discussions about this subject:

- https://inbox.sourceware.org/libc-alpha/efaffc5a404cf104f225c26dbc96e0001cede8f9.1747399542.git.alx@kernel.org/T/

- https://sourceware.org/pipermail/libc-alpha/2025-May/166675.html

- https://lkml.rescloud.iu.edu/hypermail/linux/kernel/2205.3/06731.html

- https://lwn.net/Articles/576478/

- https://yarchive.net/comp/linux/must_check.html

- https://sourceware.org/pipermail/libc-alpha/2025-May/166907.html

- https://sourceware.org/pipermail/libc-alpha/2025-May/166722.html


POSIX.1-2024 also allows EINPROGRESS after the descriptor has been closed.
I considered using that result, but it appears less useful than success
for Linux. It would preserve diagnostic information about interrupted
close-time work, but there is no operation the caller can perform on the
original fd to resume or complete that work. It would therefore turn an
irrevocably completed ownership transfer into an apparent failure without
providing a recovery path. Returning success avoids that ambiguity and
still leaves genuinely useful delayed-I/O errors such as EIO, ENOSPC, and
EDQUOT untouched.

This also matches the direction taken by musl, which initially used
EINPROGRESS for this case and later changed to success because existing
applications were prone to interpret EINPROGRESS as a failure and could
incorrectly infer that the fd was still open.

Automatically replacing EINTR with success does change the *observable*
raw syscall ABI for applications that distinguish EINTR from successful
close(). For applications that already treat the descriptor as consumed,
this changes control flow to the normal successful-close path. I would be
particularly interested in concrete examples where distinguishing EINTR
provides useful recovery semantics, given that the original fd has
already been consumed and cannot be used to resume the interrupted
close-time work.

Mikko Rantalainen (1):
  fs: don't return EINTR from close()

 fs/open.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

-- 
2.43.0

[RFC PATCH 1/1] fs: don't return EINTR from close()

[Mikko Rantalainen]

close() removes the file descriptor from the descriptor table before
calling filp_flush().  Consequently, once file_close_fd() succeeds,
the descriptor is closed regardless of the result returned by
filp_flush() and the same fd number may immediately be reused.

Without this patch an interruptible ->flush() can nevertheless cause
close() to return EINTR, either directly or after an internal -ERESTART*
error is translated to EINTR.

This is particularly problematic for close().  EINTR conventionally
indicates an interrupted operation that may need to be retried, but
retrying close() is unsafe: another thread may already have reused the
descriptor number, causing the retry to close an unrelated file.

There is also no recovery operation the caller can perform through the
original descriptor, since it has already been removed from the
descriptor table.

Treat interruption after descriptor removal as successful close instead.
Continue to report other errors from ->flush(), such as delayed I/O
errors.

This also makes this case compatible with POSIX.1-2024.  POSIX permits
an interrupted close() that has closed the descriptor to return success,
whereas if close() reports EINTR, POSIX requires the descriptor to
remain open.

musl and Android bionic already normalize EINTR from Linux close() to
success in userspace, providing substantial deployed precedent for this
behavior.

Signed-off-by: Mikko Rantalainen <mikko.rantalainen@peda.net>
---
 fs/open.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 408925d7bd0b..81a43b6c5b6b 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1513,12 +1513,17 @@ SYSCALL_DEFINE1(close, unsigned int, fd)
 	if (likely(retval == 0))
 		return 0;
 
-	/* can't restart close syscall because file table entry was cleared */
-	if (retval == -ERESTARTSYS ||
+	/*
+	 * The file descriptor has already been closed, so an interrupted
+	 * close cannot be restarted safely. Do not report EINTR after the
+	 * descriptor has been detached.
+	 */
+	if (retval == -EINTR ||
+	    retval == -ERESTARTSYS ||
 	    retval == -ERESTARTNOINTR ||
 	    retval == -ERESTARTNOHAND ||
 	    retval == -ERESTART_RESTARTBLOCK)
-		retval = -EINTR;
+		retval = 0;
 
 	return retval;
 }
-- 
2.43.0

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Rich Felker]

On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
> This is an RFC because it deliberately changes a long-established raw
> syscall ABI. Jan Kara raised userspace-regression concerns when this was
> discussed in 2025:
> 
>     https://lore.kernel.org/linux-fsdevel/ddqmhjc2rpzk2jjvunbt3l3eukcn4xzkocqzdg3j4msihdhzko@fizekvxndg2d/
> 
> while musl and Android bionic already normalize this result to success
> in libc.
> 
> The Linux implementation of close() will always close the file descriptor
> given as argument, except for the invalid file descriptor which will
> return -EBADF.
> 
> However, currently Linux kernel will return EINTR in some cases for
> close(). There is no good way for caller to recover from this case using
> the original fd. Whatever action was actually interrupted cannot be
> resumed or retried through this fd, because the fd has already been
> consumed. Even worse, EINTR conventionally invites retrying an operation,
> but retrying close() is unsafe: the same file descriptor number may
> already refer to another file opened by another thread by the time
> close() returns EINTR.
> 
> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
> descriptor must remain open. It also explicitly permits an interrupted
> close() to return success after closing the descriptor.
> 
> This patch is about implementing the second option to be compatible with
> both POSIX.1-2024 and real-world applications.
> 
> Since close() on Linux first relinquishes the file descriptor and only
> then performs ->flush() work, interruption of that later work cannot be
> recovered through the original fd. No matter how important that
> close-time work was, ownership of the fd has already been irrevocably
> relinquished.
> 
> For regular files, applications requiring durability already need an
> explicit synchronization operation such as fsync() or fdatasync() before
> relinquishing the fd. This proposal does not suppress meaningful
> delayed-I/O errors such as EIO, ENOSPC, or EDQUOT; it only changes
> interruption results whose conventional recovery action (retrying the
> operation) is unsafe for close().
> 
> Historical discussions about this subject:
> 
> - https://inbox.sourceware.org/libc-alpha/efaffc5a404cf104f225c26dbc96e0001cede8f9.1747399542.git.alx@kernel.org/T/
> 
> - https://sourceware.org/pipermail/libc-alpha/2025-May/166675.html
> 
> - https://lkml.rescloud.iu.edu/hypermail/linux/kernel/2205.3/06731.html
> 
> - https://lwn.net/Articles/576478/
> 
> - https://yarchive.net/comp/linux/must_check.html
> 
> - https://sourceware.org/pipermail/libc-alpha/2025-May/166907.html
> 
> - https://sourceware.org/pipermail/libc-alpha/2025-May/166722.html
> 
> 
> POSIX.1-2024 also allows EINPROGRESS after the descriptor has been closed.
> I considered using that result, but it appears less useful than success
> for Linux. It would preserve diagnostic information about interrupted
> close-time work, but there is no operation the caller can perform on the
> original fd to resume or complete that work. It would therefore turn an
> irrevocably completed ownership transfer into an apparent failure without
> providing a recovery path. Returning success avoids that ambiguity and
> still leaves genuinely useful delayed-I/O errors such as EIO, ENOSPC, and
> EDQUOT untouched.
> 
> This also matches the direction taken by musl, which initially used
> EINPROGRESS for this case and later changed to success because existing
> applications were prone to interpret EINPROGRESS as a failure and could
> incorrectly infer that the fd was still open.
> 
> Automatically replacing EINTR with success does change the *observable*
> raw syscall ABI for applications that distinguish EINTR from successful
> close(). For applications that already treat the descriptor as consumed,
> this changes control flow to the normal successful-close path. I would be
> particularly interested in concrete examples where distinguishing EINTR
> provides useful recovery semantics, given that the original fd has
> already been consumed and cannot be used to resume the interrupted
> close-time work.
> 
> Mikko Rantalainen (1):
>   fs: don't return EINTR from close()
> 
>  fs/open.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> -- 
> 2.43.0

Hi! I'm one of the first people who pressed this issue while tracking
down the POSIX model for how side effects are supposed to work with
respect to EINTR and how that relates to thread cancellation, and how
glibc was getting all this stuff wrong, back around 2011-2012.

I don't think there is serious concern about userspace regressions
making this change. It would not be changing the meaning of any
existing result code or adding a new error condition applications need
to be aware of (like the EINPROGRESS mess).

But I'm also not sure how helpful the change would be. It's already
possible to patch this up in userspace, and as you noted, we already
do that in musl and so does Bionic. So the main practical effect of
this change would be just forcing the right behavior on glibc systems
even when glibc doesn't want to fix it. Maybe that's a good idea? I'm
not sure. I think it would be best to have everyone on the same page
that this should be fixed, with both glibc fixing it so it's right on
old-kernel/new-glibc, and the kernel fixing it so it's right on
new-kernel/old-glibc. That would also avoid hard feelings from a
unilateral action perceived as dictatorial.

Rich

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Alejandro Colomar]

[-- Attachment #1: Type: text/plain, Size: 6008 bytes --]

> Date: 2026-09-13 16:53:58-0400
> From: Rich Felker <dalias@libc.org>
>
> On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
> > This is an RFC because it deliberately changes a long-established raw
> > syscall ABI. Jan Kara raised userspace-regression concerns when this was
> > discussed in 2025:
> > 
> >     https://lore.kernel.org/linux-fsdevel/ddqmhjc2rpzk2jjvunbt3l3eukcn4xzkocqzdg3j4msihdhzko@fizekvxndg2d/
> > 
> > while musl and Android bionic already normalize this result to success
> > in libc.
> > 
> > The Linux implementation of close() will always close the file descriptor
> > given as argument, except for the invalid file descriptor which will
> > return -EBADF.
> > 
> > However, currently Linux kernel will return EINTR in some cases for
> > close(). There is no good way for caller to recover from this case using
> > the original fd. Whatever action was actually interrupted cannot be
> > resumed or retried through this fd, because the fd has already been
> > consumed. Even worse, EINTR conventionally invites retrying an operation,
> > but retrying close() is unsafe: the same file descriptor number may
> > already refer to another file opened by another thread by the time
> > close() returns EINTR.
> > 
> > In addition, POSIX.1-2024 requires that if close() reports EINTR, the
> > descriptor must remain open. It also explicitly permits an interrupted
> > close() to return success after closing the descriptor.
> > 
> > This patch is about implementing the second option to be compatible with
> > both POSIX.1-2024 and real-world applications.
> > 
> > Since close() on Linux first relinquishes the file descriptor and only
> > then performs ->flush() work, interruption of that later work cannot be
> > recovered through the original fd. No matter how important that
> > close-time work was, ownership of the fd has already been irrevocably
> > relinquished.
> > 
> > For regular files, applications requiring durability already need an
> > explicit synchronization operation such as fsync() or fdatasync() before
> > relinquishing the fd. This proposal does not suppress meaningful
> > delayed-I/O errors such as EIO, ENOSPC, or EDQUOT; it only changes
> > interruption results whose conventional recovery action (retrying the
> > operation) is unsafe for close().
> > 
> > Historical discussions about this subject:
> > 
> > - https://inbox.sourceware.org/libc-alpha/efaffc5a404cf104f225c26dbc96e0001cede8f9.1747399542.git.alx@kernel.org/T/
> > 
> > - https://sourceware.org/pipermail/libc-alpha/2025-May/166675.html
> > 
> > - https://lkml.rescloud.iu.edu/hypermail/linux/kernel/2205.3/06731.html
> > 
> > - https://lwn.net/Articles/576478/
> > 
> > - https://yarchive.net/comp/linux/must_check.html
> > 
> > - https://sourceware.org/pipermail/libc-alpha/2025-May/166907.html
> > 
> > - https://sourceware.org/pipermail/libc-alpha/2025-May/166722.html
> > 
> > 
> > POSIX.1-2024 also allows EINPROGRESS after the descriptor has been closed.
> > I considered using that result, but it appears less useful than success
> > for Linux. It would preserve diagnostic information about interrupted
> > close-time work, but there is no operation the caller can perform on the
> > original fd to resume or complete that work. It would therefore turn an
> > irrevocably completed ownership transfer into an apparent failure without
> > providing a recovery path. Returning success avoids that ambiguity and
> > still leaves genuinely useful delayed-I/O errors such as EIO, ENOSPC, and
> > EDQUOT untouched.
> > 
> > This also matches the direction taken by musl, which initially used
> > EINPROGRESS for this case and later changed to success because existing
> > applications were prone to interpret EINPROGRESS as a failure and could
> > incorrectly infer that the fd was still open.
> > 
> > Automatically replacing EINTR with success does change the *observable*
> > raw syscall ABI for applications that distinguish EINTR from successful
> > close(). For applications that already treat the descriptor as consumed,
> > this changes control flow to the normal successful-close path. I would be
> > particularly interested in concrete examples where distinguishing EINTR
> > provides useful recovery semantics, given that the original fd has
> > already been consumed and cannot be used to resume the interrupted
> > close-time work.
> > 
> > Mikko Rantalainen (1):
> >   fs: don't return EINTR from close()
> > 
> >  fs/open.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > -- 
> > 2.43.0
> 
> Hi! I'm one of the first people who pressed this issue while tracking
> down the POSIX model for how side effects are supposed to work with
> respect to EINTR and how that relates to thread cancellation, and how
> glibc was getting all this stuff wrong, back around 2011-2012.
> 
> I don't think there is serious concern about userspace regressions
> making this change. It would not be changing the meaning of any
> existing result code or adding a new error condition applications need
> to be aware of (like the EINPROGRESS mess).
> 
> But I'm also not sure how helpful the change would be. It's already
> possible to patch this up in userspace, and as you noted, we already
> do that in musl and so does Bionic. So the main practical effect of
> this change would be just forcing the right behavior on glibc systems
> even when glibc doesn't want to fix it. Maybe that's a good idea? I'm
> not sure. I think it would be best to have everyone on the same page
> that this should be fixed, with both glibc fixing it so it's right on
> old-kernel/new-glibc, and the kernel fixing it so it's right on
> new-kernel/old-glibc. That would also avoid hard feelings from a
> unilateral action perceived as dictatorial.

Acked-by: Alejandro Colomar <alx@kernel.org>

> 
> Rich

-- 
<https://www.alejandro-colomar.es>

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Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Mikko Rantalainen]

Rich Felker (2026-09-13 23:53 Europe/Helsinki):
> On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
>> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
>> descriptor must remain open. It also explicitly permits an interrupted
>> close() to return success after closing the descriptor.
>>
>> This patch is about implementing the second option to be compatible with
>> both POSIX.1-2024 and real-world applications.
> ...
> 
> I don't think there is serious concern about userspace regressions
> making this change. It would not be changing the meaning of any
> existing result code or adding a new error condition applications need
> to be aware of (like the EINPROGRESS mess).
> 
> But I'm also not sure how helpful the change would be. It's already
> possible to patch this up in userspace, and as you noted, we already
> do that in musl and so does Bionic. So the main practical effect of
> this change would be just forcing the right behavior on glibc systems
> even when glibc doesn't want to fix it. Maybe that's a good idea? I'm
> not sure. I think it would be best to have everyone on the same page
> that this should be fixed, with both glibc fixing it so it's right on
> old-kernel/new-glibc, and the kernel fixing it so it's right on
> new-kernel/old-glibc. That would also avoid hard feelings from a
> unilateral action perceived as dictatorial.

I think there are two important questions:


(1) What is the caller truly expected to do for EINTR? They cannot
retry which would be the typical response to EINTR.

(2) If caller cannot retry because file was already closed, can caller
have any valuable information from EINTR return instead of success?
I think it's pretty safe assumption that success from close() would
be handled correctly by the caller.


I'm currently thinking that the answer to (1) is "???" and answer to
(2) is best you can do is to log "Some unknown action interrupted
some unknown background process while closing file XYZ, pretending
everything went well." because anything else would assume some
specific implementation of close() on Linux.

As I see it, the fact that avoiding EINTR return value when the file
is already closed fixing POSIX.1-2024 compatibility and glibc is just
a bonus for a change that makes sense otherwise, too.

-- 
Mikko

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Alejandro Colomar]

[-- Attachment #1: Type: text/plain, Size: 3190 bytes --]

Hi Mikko,

> Date: 2026-09-14 09:36:58+0300
> From: Mikko Rantalainen <mikko.rantalainen@peda.net>
>
> Rich Felker (2026-09-13 23:53 Europe/Helsinki):
> > On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
> >> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
> >> descriptor must remain open. It also explicitly permits an interrupted
> >> close() to return success after closing the descriptor.
> >>
> >> This patch is about implementing the second option to be compatible with
> >> both POSIX.1-2024 and real-world applications.
> > ...
> > 
> > I don't think there is serious concern about userspace regressions
> > making this change. It would not be changing the meaning of any
> > existing result code or adding a new error condition applications need
> > to be aware of (like the EINPROGRESS mess).
> > 
> > But I'm also not sure how helpful the change would be. It's already
> > possible to patch this up in userspace, and as you noted, we already
> > do that in musl and so does Bionic. So the main practical effect of
> > this change would be just forcing the right behavior on glibc systems
> > even when glibc doesn't want to fix it. Maybe that's a good idea? I'm
> > not sure. I think it would be best to have everyone on the same page
> > that this should be fixed, with both glibc fixing it so it's right on
> > old-kernel/new-glibc, and the kernel fixing it so it's right on
> > new-kernel/old-glibc. That would also avoid hard feelings from a
> > unilateral action perceived as dictatorial.
> 
> I think there are two important questions:
> 
> 
> (1) What is the caller truly expected to do for EINTR? They cannot
> retry which would be the typical response to EINTR.
> 
> (2) If caller cannot retry because file was already closed, can caller
> have any valuable information from EINTR return instead of success?
> I think it's pretty safe assumption that success from close() would
> be handled correctly by the caller.

There's one thing a caller might do:

Let's say I have a program doing a backup, and close(2) reports EINTR.
I might decide that I don't trust any of what happened, and start the
backup again from scratch.

Of course, I should have sync(2)ed before calling close(2).  So this
should be a non-issue in the first place.

This is the one thing that makes me think it might be better to keep
close(2) broken: it would be a reminder that people should flush
everything before calling close(2) if they care about their files.


Have a lovely day!
Alex

> 
> 
> I'm currently thinking that the answer to (1) is "???" and answer to
> (2) is best you can do is to log "Some unknown action interrupted
> some unknown background process while closing file XYZ, pretending
> everything went well." because anything else would assume some
> specific implementation of close() on Linux.
> 
> As I see it, the fact that avoiding EINTR return value when the file
> is already closed fixing POSIX.1-2024 compatibility and glibc is just
> a bonus for a change that makes sense otherwise, too.
> 
> -- 
> Mikko
> 

-- 
<https://www.alejandro-colomar.es>

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Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Matthew Wilcox]

On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
> However, currently Linux kernel will return EINTR in some cases for
> close(). There is no good way for caller to recover from this case using
> the original fd. Whatever action was actually interrupted cannot be
> resumed or retried through this fd, because the fd has already been
> consumed. Even worse, EINTR conventionally invites retrying an operation,
> but retrying close() is unsafe: the same file descriptor number may
> already refer to another file opened by another thread by the time
> close() returns EINTR.
> 
> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
> descriptor must remain open. It also explicitly permits an interrupted
> close() to return success after closing the descriptor.

I think any filesystem / device driver / ... which returns -EINTR from
close() is broken.  There is one exception though -- if the signal is
fatal.  It's like read()/write() being killable; if the signal is fatal,
the task dies before it gets to see the errno.  So it doesn't matter.

So that's my preferred solution; track down the bad kernel code that's
doing things in close() that are "interruptible" and convert them to
"killable".  We don't want SIGWINCH or SIGALRM interrupting close();
that's just dumb.

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Rich Felker]

On Sun, Sep 13, 2026 at 11:42:23PM +0100, Matthew Wilcox wrote:
> On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
> > However, currently Linux kernel will return EINTR in some cases for
> > close(). There is no good way for caller to recover from this case using
> > the original fd. Whatever action was actually interrupted cannot be
> > resumed or retried through this fd, because the fd has already been
> > consumed. Even worse, EINTR conventionally invites retrying an operation,
> > but retrying close() is unsafe: the same file descriptor number may
> > already refer to another file opened by another thread by the time
> > close() returns EINTR.
> > 
> > In addition, POSIX.1-2024 requires that if close() reports EINTR, the
> > descriptor must remain open. It also explicitly permits an interrupted
> > close() to return success after closing the descriptor.
> 
> I think any filesystem / device driver / ... which returns -EINTR from
> close() is broken.  There is one exception though -- if the signal is
> fatal.  It's like read()/write() being killable; if the signal is fatal,
> the task dies before it gets to see the errno.  So it doesn't matter.
> 
> So that's my preferred solution; track down the bad kernel code that's
> doing things in close() that are "interruptible" and convert them to
> "killable".

The traditional things that admit EINTR in close are bad NFS
implementations (which intentionally use interruptible instead of
killable because network flaky) and tape drives (which at least
historically performed rewinding as part of the close operation).

> We don't want SIGWINCH or SIGALRM interrupting close();
> that's just dumb.

You don't want a slow close blocking delivery of SIGALRM either.
Freezing the process during a long close is not the answer. The answer
is just getting rid of long close and having it behave properly as a
resource-handle-free-only operation and return immediately. But that's
a more invasive change than just replacing -EINTR with 0 if/when it
gets interrupted.

Rich

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Mikko Rantalainen]

Rich Felker (2026-09-14 02:51 Europe/Helsinki):
> On Sun, Sep 13, 2026 at 11:42:23PM +0100, Matthew Wilcox wrote:
>> On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
>>> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
>>> descriptor must remain open. It also explicitly permits an interrupted
>>> close() to return success after closing the descriptor.
>>
>> I think any filesystem / device driver / ... which returns -EINTR from
>> close() is broken.  There is one exception though -- if the signal is
>> fatal.  It's like read()/write() being killable; if the signal is fatal,
>> the task dies before it gets to see the errno.  So it doesn't matter.
>>
>> So that's my preferred solution; track down the bad kernel code that's
>> doing things in close() that are "interruptible" and convert them to
>> "killable".
> 
> The traditional things that admit EINTR in close are bad NFS
> implementations (which intentionally use interruptible instead of
> killable because network flaky) and tape drives (which at least
> historically performed rewinding as part of the close operation).
> 
>> We don't want SIGWINCH or SIGALRM interrupting close();
>> that's just dumb.
> 
> You don't want a slow close blocking delivery of SIGALRM either.
> Freezing the process during a long close is not the answer. The answer
> is just getting rid of long close and having it behave properly as a
> resource-handle-free-only operation and return immediately. But that's
> a more invasive change than just replacing -EINTR with 0 if/when it
> gets interrupted.


For ordinary filesystems where close-time work is not itself part of a
stronger synchronization contract, I think the answer should be fairly
clear: close() should not expose EINTR to userspace after consuming the
fd.

close() does not provide a durability guarantee. It *may* report
deferred I/O errors or errors discovered during close-time work, but
an application which requires synchronization has an explicit
interface such as fsync() or fdatasync() for that purpose.

So if such close-time work happens to be interrupted, I don't see what
useful recovery semantics exposing EINTR provides. The fd is already
gone and close() cannot safely be retried. Returning success seems more
useful than exposing an interruption which the caller cannot act on.

NFS and devices make the question more interesting, though. Some
implementations put meaningful state transitions into their close path.
Tape devices are an obvious historical example: depending on the
device/mode, close may write filemarks, flush tape buffers, rewind, etc.
Those operations are not necessarily replaceable by fsync() before
close().

But I think EINTR is problematic there too, for almost the opposite
reason. If the semantics require some close-time operation to complete,
then returning EINTR after the fd has already been removed leaves
userspace with no generic way to complete that operation:

  close(fd) -> EINTR

    - fd is already closed on Linux
    - retrying close(fd) is unsafe
    - the interrupted operation cannot be resumed through fd

That seems to leave a few possible designs:

1. The close-time operation is required. Then the kernel should complete
   it despite non-fatal signals, or provide some explicit operation which
   userspace can use to request/check completion.

2. The close-time operation is best-effort. Then interruption should not
   turn an already completed descriptor release into EINTR; returning
   success seems more appropriate.

3. POSIX.1-2024 also provides EINPROGRESS for the case where the fd has
   been closed but close-time work is incomplete. That at least describes
   the state correctly, although I still don't see what useful recovery
   operation userspace has if the work can only be performed through the
   now-consumed fd. In addition, the implementation in musl suggests
   that this wouldn't be compatible with real-world programs.

What seems particularly difficult to justify is the current Linux result:

  close(fd) -> EINTR

while the fd has nevertheless been irrevocably consumed. It has the
usual appearance of a retryable interruption without providing any safe
way to retry it.

So replacing -EINTR with 0 still looks attractive to me as the generic
close() behavior, even if removing all long synchronous close operations
would be a larger project.

It may also make sense independently to diagnose ->flush()
implementations which return an interruption result in cases where that
means required close-time work has been abandoned. I would be interested
in whether there is a good way to warn or trace those cases without
complaining about the fatal-signal case Matthew mentioned.

--
Mikko

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Mikko Rantalainen]

Matthew Wilcox (2026-09-14 01:42 Europe/Helsinki):
> On Sun, Sep 13, 2026 at 10:38:14PM +0300, Mikko Rantalainen wrote:
>> However, currently Linux kernel will return EINTR in some cases for
>> close(). There is no good way for caller to recover from this case using
>> the original fd. Whatever action was actually interrupted cannot be
>> resumed or retried through this fd, because the fd has already been
>> consumed. Even worse, EINTR conventionally invites retrying an operation,
>> but retrying close() is unsafe: the same file descriptor number may
>> already refer to another file opened by another thread by the time
>> close() returns EINTR.
>>
>> In addition, POSIX.1-2024 requires that if close() reports EINTR, the
>> descriptor must remain open. It also explicitly permits an interrupted
>> close() to return success after closing the descriptor.
> 
> I think any filesystem / device driver / ... which returns -EINTR from
> close() is broken.  There is one exception though -- if the signal is
> fatal.  It's like read()/write() being killable; if the signal is fatal,
> the task dies before it gets to see the errno.  So it doesn't matter.
> 
> So that's my preferred solution; track down the bad kernel code that's
> doing things in close() that are "interruptible" and convert them to
> "killable".  We don't want SIGWINCH or SIGALRM interrupting close();
> that's just dumb.

Am I reading this correctly as a proposed VFS invariant: ->flush() must
not return an interruption result which could become visible as EINTR
to a surviving userspace caller of close()? A fatal signal is the
exception because the task will die before observing the return value.

That seems like a reasonable invariant, but I couldn't find it
documented anywhere. Documentation/filesystems/vfs.rst currently says
only:

    flush
        called by the close(2) system call to flush a file

without specifying allowed return values or signal semantics.

I had originally approached this from the userspace contract. close(2)
guarantees relinquishing the descriptor, but does not provide a general
synchronization guarantee. In particular, successful close() does not
mean regular-file data has reached storage. Applications which require
that guarantee need an explicit synchronization operation such as
fsync() or fdatasync() before close().

The close(2) documentation does say that later close-time operations,
including flushing data to a filesystem or device, *can* report errors.
But I don't read it as guaranteeing that all such work completes or that
all outstanding errors are discovered before close() returns.

That's why I was thinking EINTR should simply be converted to success
by close() implementation.

So I think the important distinction is:

1. userspace is not generally promised completion of arbitrary
   close-time work; but

2. if a subsystem deliberately performs synchronous work in ->flush(),
   the kernel may nevertheless require that work to complete for the
   subsystem's own semantics.

If (2) is the intended VFS rule, then I agree that converting EINTR to
success in close() would hide a bug rather than fix it. The bug would
be an ->flush() implementation allowing an ordinary signal to abandon
work which it intended to perform synchronously.

Would it make sense to document that invariant explicitly, e.g. that
->flush() must not return -EINTR or -ERESTART* due to an ordinary
non-fatal signal?

If those results should be considered implementation bugs, perhaps a
useful diagnostic would also be something along these lines:

---
retval = filp_flush(file, current->files);

WARN_ONCE(retval == -EINTR ||
          retval == -ERESTARTSYS ||
          retval == -ERESTARTNOINTR ||
          retval == -ERESTARTNOHAND ||
          retval == -ERESTART_RESTARTBLOCK,
          "close: ->flush %ps returned interrupt error %d\n",
          file->f_op->flush, retval);
---

That would leave the existing userspace ABI unchanged while making
remaining offending implementations easier to find and fix.

I also considered retrying filp_flush() inside close(), but I don't
think that can be done generically. ->flush() is not documented as
safe to restart from the beginning after partial execution, and
an interruptible wait could immediately encounter the same
still-pending signal again. So fixing the interruptibility at the
offending wait seems safer if the above invariant is indeed
the intended one.

-- 
Mikko

Re: [RFC PATCH 0/1] close(): stop exposing non-retryable EINTR

[Mikko Rantalainen]

Mikko Rantalainen (2026-09-14 12:07 Europe/Helsinki):
> ---
> retval = filp_flush(file, current->files);
> 
> WARN_ONCE(retval == -EINTR ||
>           retval == -ERESTARTSYS ||
>           retval == -ERESTARTNOINTR ||
>           retval == -ERESTARTNOHAND ||
>           retval == -ERESTART_RESTARTBLOCK,
>           "close: ->flush %ps returned interrupt error %d\n",
>           file->f_op->flush, retval);
> ---
> 
> That would leave the existing userspace ABI unchanged while making
> remaining offending implementations easier to find and fix.
> 
> I also considered retrying filp_flush() inside close(), but I don't
> think that can be done generically. ->flush() is not documented as
> safe to restart from the beginning after partial execution, and
> an interruptible wait could immediately encounter the same
> still-pending signal again. So fixing the interruptibility at the
> offending wait seems safer if the above invariant is indeed
> the intended one.

Another thing I noticed is that there are already several paths where
the kernel calls filp_close() and intentionally ignores its return value.

For example, close_files() does:

    filp_close(file, files);

without checking the result. The same is true for do_close_on_exec(),
and close_range() explicitly says:

    Currently, errors to close a given file descriptor are ignored.

So I don't think a ->flush() implementation can rely on returning EINTR
and having somebody retry the interrupted operation. There are valid
close paths where nobody will ever see that return value, even when the
process itself continues running.

This seems to strengthen Matthew's point: if some work performed by
->flush() is required for correctness, that work has to tolerate these
close paths without depending on userspace retry. Returning an
interruption result cannot be the recovery mechanism.

I'm therefore leaning towards treating an observable -EINTR/-ERESTART*
from ->flush() as suspicious in general, rather than just special-casing
the close(2) syscall. The fatal-signal case is harmless because the task
will not observe the result, but close-on-exec and close_range() show
that unobserved filp_close() errors are already part of normal operation
as well.

That also makes me think documenting the intended ->flush() contract
would be useful: if required close-time work must not depend on the
caller retrying filp_close(), that seems like an important invariant for
implementations to know.

What guarantees must file_operations::flush provide when its caller may
have no way to act on its return value?

In any case, I'm now thinking that returning EINTR for close() is a bug
when file descriptor is already freed. I think the only question is how
it should be solved. I initially thought it should just be mapped to
success. Maybe it should be logged as subsystem bug *and* mapped to
success for userspace instead?

-- 
Mikko