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    Subject[tip:sched/urgent] sched_clock: Fix atomicity/continuity bug by using cmpxchg64()
    Commit-ID:  c7eca501d2ac092f2f58452a6f962249e2ab5dbe
    Gitweb: http://git.kernel.org/tip/c7eca501d2ac092f2f58452a6f962249e2ab5dbe
    Author: Eric Dumazet <eric.dumazet@gmail.com>
    AuthorDate: Wed, 30 Sep 2009 20:36:19 +0200
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Wed, 30 Sep 2009 22:51:51 +0200

    sched_clock: Fix atomicity/continuity bug by using cmpxchg64()

    Commit def0a9b2573 (sched_clock: Make it NMI safe) assumed
    cmpxchg() of 64bit values was available on X86_32.

    That is not so - and causes some subtle scheduler misbehavior due
    to incorrect timestamps off to up by ~4 seconds.

    Two symptoms are known right now:

    - interactivity problems seen by Arjan: up to 600 msecs
    latencies instead of the expected 20-40 msecs. These
    latencies are very visible on the desktop.

    - incorrect CPU stats: occasionally too high percentages in 'top',
    and crazy CPU usage stats.

    Reported-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
    Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
    Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: John Stultz <johnstul@us.ibm.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <20090930170754.0886ff2e@infradead.org>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    kernel/sched_clock.c | 4 ++--
    1 files changed, 2 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
    index ac2e1dc..479ce56 100644
    --- a/kernel/sched_clock.c
    +++ b/kernel/sched_clock.c
    @@ -127,7 +127,7 @@ again:
    clock = wrap_max(clock, min_clock);
    clock = wrap_min(clock, max_clock);

    - if (cmpxchg(&scd->clock, old_clock, clock) != old_clock)
    + if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
    goto again;

    return clock;
    @@ -163,7 +163,7 @@ again:
    val = remote_clock;
    }

    - if (cmpxchg(ptr, old_val, val) != old_val)
    + if (cmpxchg64(ptr, old_val, val) != old_val)
    goto again;

    return val;

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