[PATCH -next] locking/osq_lock: annotate a data race in osq_lock

[PATCH -next] locking/osq_lock: annotate a data race in osq_lock

[Qian Cai]

prev->next could be accessed concurrently as noticed by KCSAN,

 write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
  osq_lock+0x25f/0x350
  osq_wait_next at kernel/locking/osq_lock.c:79
  (inlined by) osq_lock at kernel/locking/osq_lock.c:185
  rwsem_optimistic_spin
  <snip>

 read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
  osq_lock+0x196/0x350
  osq_lock at kernel/locking/osq_lock.c:157
  rwsem_optimistic_spin
  <snip>

Since the write only stores NULL to prev->next and the read tests if
prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
shattered, the code is still working correctly. Thus, mark it as an
intentional data race using the data_race() macro.

Signed-off-by: Qian Cai <cai@lca.pw>
---
 kernel/locking/osq_lock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 1f7734949ac8..3c44ddbc11ce 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -154,7 +154,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 */
 
 	for (;;) {
-		if (prev->next == node &&
+		if (data_race(prev->next == node) &&
 		    cmpxchg(&prev->next, node, NULL) == node)
 			break;
 
-- 
2.21.0 (Apple Git-122.2)

Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock

[Marco Elver]

On Tue, 11 Feb 2020 at 05:07, Qian Cai <cai@lca.pw> wrote:
>
> prev->next could be accessed concurrently as noticed by KCSAN,
>
>  write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
>   osq_lock+0x25f/0x350
>   osq_wait_next at kernel/locking/osq_lock.c:79
>   (inlined by) osq_lock at kernel/locking/osq_lock.c:185
>   rwsem_optimistic_spin
>   <snip>
>
>  read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
>   osq_lock+0x196/0x350
>   osq_lock at kernel/locking/osq_lock.c:157
>   rwsem_optimistic_spin
>   <snip>
>
> Since the write only stores NULL to prev->next and the read tests if
> prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
> shattered, the code is still working correctly. Thus, mark it as an
> intentional data race using the data_race() macro.

I have said this before: we're not just guarding against load/store
tearing, although on their own, they make it deceptively easy to
reason about data races.

The case here seems to be another instance of a C-CAS, to avoid
unnecessarily dirtying a cacheline.

Here, the loop would make me suspicious, because a compiler could
optimize out re-loading the value. Due to the smp_load_acquire,
however, at the least we have 1 implied compiler barrier in this loop
which means that will likely not happen.

Before jumping to 'data_race()', I would ask again: how bad is the
READ_ONCE? Is the generated code the same? If so, just use the
READ_ONCE. Do you want to reason about all compiler optimizations? For
this code here, I certainly don't want to.

But in the end it's up to what maintainers prefer, and maybe there is
a very compelling argument that I missed that makes the fact this is a
data race always safe.

Thanks,
-- Marco

> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  kernel/locking/osq_lock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 1f7734949ac8..3c44ddbc11ce 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -154,7 +154,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>          */
>
>         for (;;) {
> -               if (prev->next == node &&
> +               if (data_race(prev->next == node) &&
>                     cmpxchg(&prev->next, node, NULL) == node)
>                         break;
>
> --
> 2.21.0 (Apple Git-122.2)
>

Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock

[Qian Cai]

> On Feb 11, 2020, at 5:16 AM, Marco Elver <elver@google.com> wrote:
> 
> I have said this before: we're not just guarding against load/store
> tearing, although on their own, they make it deceptively easy to
> reason about data races.
> 
> The case here seems to be another instance of a C-CAS, to avoid
> unnecessarily dirtying a cacheline.
> 
> Here, the loop would make me suspicious, because a compiler could
> optimize out re-loading the value. Due to the smp_load_acquire,
> however, at the least we have 1 implied compiler barrier in this loop
> which means that will likely not happen.
> 
> Before jumping to 'data_race()', I would ask again: how bad is the
> READ_ONCE? Is the generated code the same? If so, just use the
> READ_ONCE. Do you want to reason about all compiler optimizations? For
> this code here, I certainly don't want to.
> 
> But in the end it's up to what maintainers prefer, and maybe there is
> a very compelling argument that I missed that makes the fact this is a
> data race always safe.

Yes, I feel like locking maintainers prefer data_race() rather than blindly adding READ_ONCE() unless there is an strong evidence that the later is needed.

Since I can’t prove it is strictly needed to prevent from which specific optimization, I had chosen the data_race() approach.

Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock

[Peter Zijlstra]

On Tue, Feb 11, 2020 at 11:16:05AM +0100, Marco Elver wrote:
> On Tue, 11 Feb 2020 at 05:07, Qian Cai <cai@lca.pw> wrote:
> >
> > prev->next could be accessed concurrently as noticed by KCSAN,
> >
> >  write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
> >   osq_lock+0x25f/0x350
> >   osq_wait_next at kernel/locking/osq_lock.c:79
> >   (inlined by) osq_lock at kernel/locking/osq_lock.c:185
> >   rwsem_optimistic_spin
> >   <snip>
> >
> >  read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
> >   osq_lock+0x196/0x350
> >   osq_lock at kernel/locking/osq_lock.c:157
> >   rwsem_optimistic_spin
> >   <snip>
> >
> > Since the write only stores NULL to prev->next and the read tests if
> > prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
> > shattered, the code is still working correctly. Thus, mark it as an
> > intentional data race using the data_race() macro.
> 
> I have said this before: we're not just guarding against load/store
> tearing, although on their own, they make it deceptively easy to
> reason about data races.
> 
> The case here seems to be another instance of a C-CAS, to avoid
> unnecessarily dirtying a cacheline.
> 
> Here, the loop would make me suspicious, because a compiler could
> optimize out re-loading the value. Due to the smp_load_acquire,
> however, at the least we have 1 implied compiler barrier in this loop
> which means that will likely not happen.

The loop has cpu_relax() (as any spin loop should have), that implies a
compiler barrier() and should disallow the compiler from being funny.

That said; I feel it would be very good to mandate a comment with every
use of data_race(), just like we mandate a comment with memory barriers.
This comment can then explain why the data_race() annotation is correct.