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

[Peter Zijlstra <peterz@infradead.org>]

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.