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    SubjectRe: [PATCH RFC tip/core/rcu 11/20] rcu: fix race condition in synchronize_sched_expedited()

    On 12/17/2010 09:54 PM, Paul E. McKenney wrote:
    > The new (early 2010) implementation of synchronize_sched_expedited() uses
    > try_stop_cpu() to force a context switch on every CPU. It also permits
    > concurrent calls to synchronize_sched_expedited() to share a single call
    > to try_stop_cpu() through use of an atomically incremented
    > synchronize_sched_expedited_count variable. Unfortunately, this is
    > subject to failure as follows:
    > o Task A invokes synchronize_sched_expedited(), try_stop_cpus()
    > succeeds, but Task A is preempted before getting to the atomic
    > increment of synchronize_sched_expedited_count.
    > o Task B also invokes synchronize_sched_expedited(), with exactly
    > the same outcome as Task A.
    > o Task C also invokes synchronize_sched_expedited(), again with
    > exactly the same outcome as Tasks A and B.
    > o Task D also invokes synchronize_sched_expedited(), but only
    > gets as far as acquiring the mutex within try_stop_cpus()
    > before being preempted, interrupted, or otherwise delayed.
    > o Task E also invokes synchronize_sched_expedited(), but only
    > gets to the snapshotting of synchronize_sched_expedited_count.
    > o Tasks A, B, and C all increment synchronize_sched_expedited_count.
    > o Task E fails to get the mutex, so checks the new value
    > of synchronize_sched_expedited_count. It finds that the
    > value has increased, so (wrongly) assumes that its work
    > has been done, returning despite there having been no
    > expedited grace period since it began.
    > The solution is to have the lowest-numbered CPU atomically increment
    > the synchronize_sched_expedited_count variable within the
    > synchronize_sched_expedited_cpu_stop() function, which is under
    > the protection of the mutex acquired by try_stop_cpus(). However, this
    > also requires that piggybacking tasks wait for three rather than two
    > instances of try_stop_cpu(), because we cannot control the order in
    > which the per-CPU callback function occur.
    > Cc: Tejun Heo <>
    > Cc: Lai Jiangshan <>
    > Signed-off-by: Paul E. McKenney <>

    Acked-by: Tejun Heo <>

    I suppose this should go -stable?


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