Messages in this thread Patch in this message | | | Date | Sat, 9 May 2020 09:12:17 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock |
| |
On Sat, May 09, 2020 at 09:01:53AM -0400, Qian Cai wrote: > > > > On May 9, 2020, at 12:33 AM, Paul E. McKenney <paulmck@kernel.org> wrote: > > > > 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. > > Peter, Will, can you give an ACK? This v2 should incorporate all the feedback from Peter, > > https://lore.kernel.org/lkml/20200211124753.GP14914@hirez.programming.kicks-ass.net/ > > V5.9 is fine. All I care about is it is always in linux-next (so the testing bots won’t trigger this over and over again) and to be in mainline at some point in the future.
Ah, and I forgot to ask. Why "if (data_race(prev->next == node)" instead of "if (data_race(prev->next) == node"?
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 >
| |