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    Subject[PATCH v2 3/9] sched: Replace spin_unlock_wait() with lock/unlock pair
    Date
    There is no agreed-upon definition of spin_unlock_wait()'s semantics,
    and it appears that all callers could do just as well with a lock/unlock
    pair. This commit therefore replaces the spin_unlock_wait() call in
    do_task_dead() with spin_lock() followed immediately by spin_unlock().
    This should be safe from a performance perspective because the lock is
    this tasks ->pi_lock, and this is called only after the task exits.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: Alan Stern <stern@rowland.harvard.edu>
    Cc: Andrea Parri <parri.andrea@gmail.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    [ paulmck: Replace leading smp_mb() with smp_mb__before_spinlock(),
    courtesy of Arnd Bergmann's noting its odd location. ]
    ---
    kernel/sched/core.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index e91138fcde86..48a8760fedf4 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -3460,8 +3460,9 @@ void __noreturn do_task_dead(void)
    * To avoid it, we have to wait for releasing tsk->pi_lock which
    * is held by try_to_wake_up()
    */
    - smp_mb();
    - raw_spin_unlock_wait(&current->pi_lock);
    + smp_mb__before_spinlock();
    + raw_spin_lock_irq(&current->pi_lock);
    + raw_spin_unlock_irq(&current->pi_lock);

    /* Causes final put_task_struct in finish_task_switch(): */
    __set_current_state(TASK_DEAD);
    --
    2.5.2
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