[PATCH] close_files(): reimplement based on do_close_on_exec()

[PATCH] close_files(): reimplement based on do_close_on_exec()

[Mateusz Guzik]

While here take more advantage of the fact nobody should be messing with
the table anymore and don't clear the fd slot.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---

how about this instead, I think it's a nicer clean up.

It's literally do_close_on_exec except locking and put fd are deleted.

boots & does not blow up, but admittedly I did not bother with ltp or
any serious testing

 fs/file.c | 37 +++++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 16 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 74d7ad676579..3ff2e8265156 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -389,33 +389,38 @@ struct files_struct *dup_fd(struct files_struct *oldf, unsigned int max_fds)
 	return newf;
 }
 
-static struct fdtable *close_files(struct files_struct * files)
+static struct fdtable *close_files(struct files_struct *files)
 {
 	/*
 	 * It is safe to dereference the fd table without RCU or
 	 * ->file_lock because this is the last reference to the
 	 * files structure.
+	 *
+	 * For the same reason we can skip locking.
 	 */
 	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
-	unsigned int i, j = 0;
+	unsigned i;
 
-	for (;;) {
+	for (i = 0; ; i++) {
 		unsigned long set;
-		i = j * BITS_PER_LONG;
-		if (i >= fdt->max_fds)
+		unsigned fd = i * BITS_PER_LONG;
+		fdt = files_fdtable(files);
+		if (fd >= fdt->max_fds)
 			break;
-		set = fdt->open_fds[j++];
-		while (set) {
-			if (set & 1) {
-				struct file * file = xchg(&fdt->fd[i], NULL);
-				if (file) {
-					filp_close(file, files);
-					cond_resched();
-				}
-			}
-			i++;
-			set >>= 1;
+		set = fdt->open_fds[i];
+		if (!set)
+			continue;
+		for ( ; set ; fd++, set >>= 1) {
+			struct file *file;
+			if (!(set & 1))
+				continue;
+			file = fdt->fd[fd];
+			if (!file)
+				continue;
+			filp_close(file, files);
+			cond_resched();
 		}
+
 	}
 
 	return fdt;
-- 
2.43.0

Re: [PATCH] close_files(): reimplement based on do_close_on_exec()

[Al Viro]

On Mon, Aug 12, 2024 at 09:56:58AM +0200, Mateusz Guzik wrote:
> While here take more advantage of the fact nobody should be messing with
> the table anymore and don't clear the fd slot.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
> ---
> 
> how about this instead, I think it's a nicer clean up.

> It's literally do_close_on_exec except locking and put fd are deleted.

TBH, I don't see much benefit that way - if anything, you are doing
a bunch of extra READ_ONCE() of the same thing (files->fdt), for no
visible reason...

Re: [PATCH] close_files(): reimplement based on do_close_on_exec()

[Mateusz Guzik]

On Wed, Aug 14, 2024 at 7:24 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Mon, Aug 12, 2024 at 09:56:58AM +0200, Mateusz Guzik wrote:
> > While here take more advantage of the fact nobody should be messing with
> > the table anymore and don't clear the fd slot.
> >
> > Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
> > ---
> >
> > how about this instead, I think it's a nicer clean up.
>
> > It's literally do_close_on_exec except locking and put fd are deleted.
>
> TBH, I don't see much benefit that way - if anything, you are doing
> a bunch of extra READ_ONCE() of the same thing (files->fdt), for no
> visible reason...

I claim the stock code avoidably implements traversal differently from
do_close_on_exec.

The fdt reload can be trivially lifted out of the loop, does not
affect what I was going for.

But now that you mention this can also be done in the do_close_on_exec
case -- the thread calling it is supposed to be the only consumer, so
fdt can't change.

that's my $0,03 here, I'm not going to further argue about it
-- 
Mateusz Guzik <mjguzik gmail.com>