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    Subject[PATCH tip/core/rcu 05/10] locking/osq_lock: Annotate a data race in osq_lock
    Date
    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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