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    Subject[PATCH RFC tip/core/rcu 10/28] rcu: Disable preemption in rcu_is_cpu_idle()
    Date
    From: Paul E. McKenney <paul.mckenney@linaro.org>

    Because rcu_is_cpu_idle() is to be used to check for extended quiescent
    states in RCU-preempt read-side critical sections, it cannot assume that
    preemption is disabled. And preemption must be disabled when accessing
    the dyntick-idle state, because otherwise the following sequence of events
    could occur:

    1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer
    to CPU 1's per-CPU variables.

    2. Task B preempts Task A and starts running on CPU 1.

    3. Task A migrates to CPU 2.

    4. Task B blocks, leaving CPU 1 idle.

    5. Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle
    information using the pointer fetched in step 1 above, and finds
    that CPU 1 is idle.

    6. Task A therefore incorrectly concludes that it is executing in
    an extended quiescent state, possibly issuing a spurious splat.

    Therefore, this commit disables preemption within the rcu_is_cpu_idle()
    function.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcutree.c | 10 +++++++---
    1 files changed, 7 insertions(+), 3 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index abb5167..c097394 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -556,12 +556,16 @@ void rcu_nmi_exit(void)
    * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
    *
    * If the current CPU is in its idle loop and is neither in an interrupt
    - * or NMI handler, return true. The caller must have at least disabled
    - * preemption.
    + * or NMI handler, return true.
    */
    int rcu_is_cpu_idle(void)
    {
    - return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
    + int ret;
    +
    + preempt_disable();
    + ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
    + preempt_enable();
    + return ret;
    }

    #endif /* #ifdef CONFIG_PROVE_RCU */
    --
    1.7.3.2


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