Messages in this thread | | | From | Marco Elver <> | Date | Tue, 11 Feb 2020 11:16:05 +0100 | Subject | Re: [PATCH -next] locking/osq_lock: annotate a data race in osq_lock |
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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) >
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