[PATCH] Fixup for WRITE_ONCE vs. sparse

[PATCH] Fixup for WRITE_ONCE vs. sparse

[Christian Borntraeger]

here is a fixup for WRITE_ONCE dealing with rcu accesses.
I did some cross compiles as well as some sparse checkings.
I pushed this out to
git.kernel.org/cgit/linux/kernel/git/borntraeger/linux.git linux-next
so hoepfully kbuild-bot will tell if there is a problem


Christian Borntraeger (1):
  compiler.h: cast away attributes in WRITE_ONCE magic

 include/linux/compiler.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

-- 
2.3.8

[PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

[Christian Borntraeger]

kernel build bot showed a warning triggered by commit
76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
smp_store_release()/smp_load_acquire()"). Turns out that sparse
does not like WRITE_ONCE accessing elements from the (sparse)
rcu address space.

fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
fs/afs/inode.c:448:9:    expected struct afs_permits *__val
fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>

Solution is to force cast away the sparse attributes for the initializer
of the union in WRITE_SAME. As this now gets too long, lets split
the macro.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 include/linux/compiler.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e08a6ae..c836eb2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define WRITE_ONCE(x, val) \
-	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+({							\
+	union { typeof(x) __val; char __c[1]; } __u =	\
+		{ .__val = (__force typeof(x)) (val) }; \
+	__write_once_size(&(x), __u.__c, sizeof(x));	\
+	__u.__val;					\
+})
 
 /**
  * READ_ONCE_CTRL - Read a value heading a control dependency
-- 
2.3.8

Re: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

[Peter Zijlstra]

On Tue, Aug 04, 2015 at 09:55:48AM +0200, Christian Borntraeger wrote:
> kernel build bot showed a warning triggered by commit
> 76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
> smp_store_release()/smp_load_acquire()"). Turns out that sparse
> does not like WRITE_ONCE accessing elements from the (sparse)
> rcu address space.
> 
> fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
> fs/afs/inode.c:448:9:    expected struct afs_permits *__val
> fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>
> 
> Solution is to force cast away the sparse attributes for the initializer
> of the union in WRITE_SAME. As this now gets too long, lets split

WRITE_ONCE, right?

> the macro.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  include/linux/compiler.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index e08a6ae..c836eb2 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
>  
>  #define WRITE_ONCE(x, val) \
> -	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> +({							\
> +	union { typeof(x) __val; char __c[1]; } __u =	\
> +		{ .__val = (__force typeof(x)) (val) }; \
> +	__write_once_size(&(x), __u.__c, sizeof(x));	\
> +	__u.__val;					\
> +})

Thanks!

READ_ONCE() doesn't have a similar problem because it only has the one
input type, which automagically becomes the output type, right?

Re: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

[Christian Borntraeger]

Am 04.08.2015 um 11:20 schrieb Peter Zijlstra:
> On Tue, Aug 04, 2015 at 09:55:48AM +0200, Christian Borntraeger wrote:
>> kernel build bot showed a warning triggered by commit
>> 76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
>> smp_store_release()/smp_load_acquire()"). Turns out that sparse
>> does not like WRITE_ONCE accessing elements from the (sparse)
>> rcu address space.
>>
>> fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
>> fs/afs/inode.c:448:9:    expected struct afs_permits *__val
>> fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>
>>
>> Solution is to force cast away the sparse attributes for the initializer
>> of the union in WRITE_SAME. As this now gets too long, lets split
> 
> WRITE_ONCE, right?

Indeed, guess what area I was looking into before....;-)

Shall I respin or can you fixup.


> 
>> the macro.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>>  include/linux/compiler.h | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> index e08a6ae..c836eb2 100644
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>>  	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
>>  
>>  #define WRITE_ONCE(x, val) \
>> -	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
>> +({							\
>> +	union { typeof(x) __val; char __c[1]; } __u =	\
>> +		{ .__val = (__force typeof(x)) (val) }; \
>> +	__write_once_size(&(x), __u.__c, sizeof(x));	\
>> +	__u.__val;					\
>> +})
> 
> Thanks!
> 
> READ_ONCE() doesn't have a similar problem because it only has the one
> input type, which automagically becomes the output type, right?

Good question. The assigment that failed was the initialization of the temporary union,
which was introduced to handle const pointers. READ_ONCE does not have such assignment
so this thing should be safe. I cant say right now if the macro return value causes
issues somewhere else, e.g. does code want to have values __rcu tagged?

I have checked some of the kbuild output and all new things seem to be fixed so this
patch itself is probably an improvement (pending kbuild coverage). 

Having a testcase that works with ACCESS_ONCE but fails with READ_ONCE would be a good
start to see if we need to change READ_ONCE further.

Christian

Re: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

[Peter Zijlstra]

On Tue, Aug 04, 2015 at 12:33:04PM +0200, Christian Borntraeger wrote:
> Am 04.08.2015 um 11:20 schrieb Peter Zijlstra:
> > On Tue, Aug 04, 2015 at 09:55:48AM +0200, Christian Borntraeger wrote:
> >> kernel build bot showed a warning triggered by commit
> >> 76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
> >> smp_store_release()/smp_load_acquire()"). Turns out that sparse
> >> does not like WRITE_ONCE accessing elements from the (sparse)
> >> rcu address space.
> >>
> >> fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
> >> fs/afs/inode.c:448:9:    expected struct afs_permits *__val
> >> fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>
> >>
> >> Solution is to force cast away the sparse attributes for the initializer
> >> of the union in WRITE_SAME. As this now gets too long, lets split
> > 
> > WRITE_ONCE, right?
> 
> Indeed, guess what area I was looking into before....;-)
> 
> Shall I respin or can you fixup.

I've already fixed it. Thanks!

[tip:locking/core] locking, compiler.h: Cast away attributes in the WRITE_ONCE() magic

[tip-bot for Christian Borntraeger]

Commit-ID:  ba33034fffc1189d95301bd865f1c799256e72a2
Gitweb:     http://git.kernel.org/tip/ba33034fffc1189d95301bd865f1c799256e72a2
Author:     Christian Borntraeger <borntraeger@de.ibm.com>
AuthorDate: Tue, 4 Aug 2015 09:55:48 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 12 Aug 2015 11:58:58 +0200

locking, compiler.h: Cast away attributes in the WRITE_ONCE() magic

The kernel build bot showed a new warning triggered by commit:

  76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in smp_store_release()/smp_load_acquire()")

because Sparse does not like WRITE_ONCE() accessing elements
from the (sparse) RCU address space:

  fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
  fs/afs/inode.c:448:9:    expected struct afs_permits *__val
  fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>

Solution is to force cast away the sparse attributes for the initializer
of the union in WRITE_ONCE().

(And as this now gets too long, also split the macro into multiple lines.)

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1438674948-38310-2-git-send-email-borntraeger@de.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/compiler.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e08a6ae..c836eb2 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
 #define WRITE_ONCE(x, val) \
-	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
+({							\
+	union { typeof(x) __val; char __c[1]; } __u =	\
+		{ .__val = (__force typeof(x)) (val) }; \
+	__write_once_size(&(x), __u.__c, sizeof(x));	\
+	__u.__val;					\
+})
 
 /**
  * READ_ONCE_CTRL - Read a value heading a control dependency