Messages in this thread | | | Date | Fri, 8 May 2020 21:33:08 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock |
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On Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote: > > > > On Feb 11, 2020, at 8:54 AM, 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. > > > > Signed-off-by: Qian Cai <cai@lca.pw> > > Hmm, this patch has been dropped from linux-next from some reasons. > > Paul, can you pick this up along with KCSAN fixes? > > https://lore.kernel.org/lkml/1581429255-12542-1-git-send-email-cai@lca.pw/
I have queued it on -rcu, but it is too late for v5.8 via the -rcu tree, so this is v5.9 at the earliest. Plus I would need an ack from one of the locking folks.
Thanx, Paul
> > --- > > > > v2: insert some code comments. > > > > kernel/locking/osq_lock.c | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > > index 1f7734949ac8..f733bcd99e8a 100644 > > --- a/kernel/locking/osq_lock.c > > +++ b/kernel/locking/osq_lock.c > > @@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > */ > > > > for (;;) { > > - if (prev->next == node && > > + /* > > + * cpu_relax() below implies a compiler barrier which would > > + * prevent this comparison being optimized away. > > + */ > > + if (data_race(prev->next == node) && > > cmpxchg(&prev->next, node, NULL) == node) > > break; > > > > -- > > 1.8.3.1 > > >
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