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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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