From: Peter Zijlstra <email@example.com> To: Marco Elver <firstname.lastname@example.org> Cc: Qian Cai <email@example.com>, Ingo Molnar <firstname.lastname@example.org>, Will Deacon <email@example.com>, LKML <firstname.lastname@example.org> Subject: Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock Date: Tue, 11 Feb 2020 13:47:53 +0100 [thread overview] Message-ID: <20200211124753.GP14914@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <CANpmjNPWCu+w3O8cg++X4=viVFsWNehTXzTuqbwV8-DcXXpFng@mail.gmail.com> On Tue, Feb 11, 2020 at 11:16:05AM +0100, Marco Elver wrote: > On Tue, 11 Feb 2020 at 05:07, Qian Cai <email@example.com> 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.
prev parent reply other threads:[~2020-02-11 12:48 UTC|newest] Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-11 4:06 Qian Cai 2020-02-11 10:16 ` Marco Elver 2020-02-11 11:57 ` Qian Cai 2020-02-11 12:47 ` Peter Zijlstra [this message]
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