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    Subject[tip:sched/core] stop_machine: Change cpu_stop_queue_two_works() to rely on stopper->enabled
    Commit-ID:  d8bc853582bfd81a9c08ca6922aeb01570080ccc
    Gitweb: http://git.kernel.org/tip/d8bc853582bfd81a9c08ca6922aeb01570080ccc
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Thu, 8 Oct 2015 19:01:41 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Tue, 20 Oct 2015 10:23:55 +0200

    stop_machine: Change cpu_stop_queue_two_works() to rely on stopper->enabled

    Change cpu_stop_queue_two_works() to ensure that both CPU's have
    stopper->enabled == T or fail otherwise.

    This way stop_two_cpus() no longer needs to check cpu_active() to
    avoid the deadlock. This patch doesn't remove these checks, we will
    do this later.

    Note: we need to take both stopper->lock's at the same time, but this
    will also help to remove lglock from stop_machine.c, so I hope this
    is fine.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: heiko.carstens@de.ibm.com
    Link: http://lkml.kernel.org/r/20151008170141.GA25537@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/stop_machine.c | 29 ++++++++++++++++++++---------
    1 file changed, 20 insertions(+), 9 deletions(-)

    diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
    index 688d6b3..91fbb10 100644
    --- a/kernel/stop_machine.c
    +++ b/kernel/stop_machine.c
    @@ -219,12 +219,27 @@ static int multi_cpu_stop(void *data)
    static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
    int cpu2, struct cpu_stop_work *work2)
    {
    + struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
    + struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
    + int err;
    +
    lg_double_lock(&stop_cpus_lock, cpu1, cpu2);
    - cpu_stop_queue_work(cpu1, work1);
    - cpu_stop_queue_work(cpu2, work2);
    + spin_lock_irq(&stopper1->lock);
    + spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
    +
    + err = -ENOENT;
    + if (!stopper1->enabled || !stopper2->enabled)
    + goto unlock;
    +
    + err = 0;
    + __cpu_stop_queue_work(stopper1, work1);
    + __cpu_stop_queue_work(stopper2, work2);
    +unlock:
    + spin_unlock(&stopper2->lock);
    + spin_unlock_irq(&stopper1->lock);
    lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);

    - return 0;
    + return err;
    }
    /**
    * stop_two_cpus - stops two cpus
    @@ -261,12 +276,8 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
    set_state(&msdata, MULTI_STOP_PREPARE);

    /*
    - * If we observe both CPUs active we know _cpu_down() cannot yet have
    - * queued its stop_machine works and therefore ours will get executed
    - * first. Or its not either one of our CPUs that's getting unplugged,
    - * in which case we don't care.
    - *
    - * This relies on the stopper workqueues to be FIFO.
    + * We do not want to migrate to inactive CPU. FIXME: move this
    + * into migrate_swap_stop() callback.
    */
    if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
    preempt_enable();

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