Messages in this thread Patch in this message | | | From | paulmck@kernel ... | Subject | [PATCH tip/core/rcu 05/10] locking/osq_lock: Annotate a data race in osq_lock | Date | Mon, 22 Jun 2020 17:43:28 -0700 |
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From: Qian Cai <cai@lca.pw>
The prev->next pointer can 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> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> --- 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 1f77349..1de006e 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; -- 2.9.5
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