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    Subject[PATCH 4.14 011/193] sched: Make resched_cpu() unconditional
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

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

    From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    commit 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 upstream.

    The current implementation of synchronize_sched_expedited() incorrectly
    assumes that resched_cpu() is unconditional, which it is not. This means
    that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
    fails as follows (analysis by Neeraj Upadhyay):

    o CPU1 is waiting for expedited wait to complete:

    sync_rcu_exp_select_cpus
    rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
    IPI sent to CPU5

    synchronize_sched_expedited_wait
    ret = swait_event_timeout(rsp->expedited_wq,
    sync_rcu_preempt_exp_done(rnp_root),
    jiffies_stall);

    expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())

    o CPU5 handles IPI and fails to acquire rq lock.

    Handles IPI
    sync_sched_exp_handler
    resched_cpu
    returns while failing to try lock acquire rq->lock
    need_resched is not set

    o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
    idle (schedule() is not called).

    o CPU 1 reports RCU stall.

    Given that resched_cpu() is now used only by RCU, this commit fixes the
    assumption by making resched_cpu() unconditional.

    Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/sched/core.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -505,8 +505,7 @@ void resched_cpu(int cpu)
    struct rq *rq = cpu_rq(cpu);
    unsigned long flags;

    - if (!raw_spin_trylock_irqsave(&rq->lock, flags))
    - return;
    + raw_spin_lock_irqsave(&rq->lock, flags);
    resched_curr(rq);
    raw_spin_unlock_irqrestore(&rq->lock, flags);
    }

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