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    Subject[ 18/33] wait: fix false timeouts when using wait_event_timeout()
    Date
    3.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Imre Deak <imre.deak@intel.com>

    commit 4c663cfc523a88d97a8309b04a089c27dc57fd7e upstream.

    Many callers of the wait_event_timeout() and
    wait_event_interruptible_timeout() expect that the return value will be
    positive if the specified condition becomes true before the timeout
    elapses. However, at the moment this isn't guaranteed. If the wake-up
    handler is delayed enough, the time remaining until timeout will be
    calculated as 0 - and passed back as a return value - even if the
    condition became true before the timeout has passed.

    Fix this by returning at least 1 if the condition becomes true. This
    semantic is in line with what wait_for_condition_timeout() does; see
    commit bb10ed09 ("sched: fix wait_for_completion_timeout() spurious
    failure under heavy load").

    Daniel said "We have 3 instances of this bug in drm/i915. One case even
    where we switch between the interruptible and not interruptible
    wait_event_timeout variants, foolishly presuming they have the same
    semantics. I very much like this."

    One such bug is reported at
    https://bugs.freedesktop.org/show_bug.cgi?id=64133

    Signed-off-by: Imre Deak <imre.deak@intel.com>
    Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Acked-by: David Howells <dhowells@redhat.com>
    Acked-by: Jens Axboe <axboe@kernel.dk>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Cc: Dave Jones <davej@redhat.com>
    Cc: Lukas Czerner <lczerner@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/wait.h | 16 +++++++++++-----
    1 file changed, 11 insertions(+), 5 deletions(-)

    --- a/include/linux/wait.h
    +++ b/include/linux/wait.h
    @@ -233,6 +233,8 @@ do { \
    if (!ret) \
    break; \
    } \
    + if (!ret && (condition)) \
    + ret = 1; \
    finish_wait(&wq, &__wait); \
    } while (0)

    @@ -249,8 +251,9 @@ do { \
    * wake_up() has to be called after changing any variable that could
    * change the result of the wait condition.
    *
    - * The function returns 0 if the @timeout elapsed, and the remaining
    - * jiffies if the condition evaluated to true before the timeout elapsed.
    + * The function returns 0 if the @timeout elapsed, or the remaining
    + * jiffies (at least 1) if the @condition evaluated to %true before
    + * the @timeout elapsed.
    */
    #define wait_event_timeout(wq, condition, timeout) \
    ({ \
    @@ -318,6 +321,8 @@ do { \
    ret = -ERESTARTSYS; \
    break; \
    } \
    + if (!ret && (condition)) \
    + ret = 1; \
    finish_wait(&wq, &__wait); \
    } while (0)

    @@ -334,9 +339,10 @@ do { \
    * wake_up() has to be called after changing any variable that could
    * change the result of the wait condition.
    *
    - * The function returns 0 if the @timeout elapsed, -ERESTARTSYS if it
    - * was interrupted by a signal, and the remaining jiffies otherwise
    - * if the condition evaluated to true before the timeout elapsed.
    + * Returns:
    + * 0 if the @timeout elapsed, -%ERESTARTSYS if it was interrupted by
    + * a signal, or the remaining jiffies (at least 1) if the @condition
    + * evaluated to %true before the @timeout elapsed.
    */
    #define wait_event_interruptible_timeout(wq, condition, timeout) \
    ({ \



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