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    Subject[RFC PATCH 3/4] Cleanup: sched/membarrier: only sync_core before usermode for same mm
    Date
    When the prev and next task's mm change, switch_mm() provides the core
    serializing guarantees before returning to usermode. The only case
    where an explicit core serialization is needed is when the scheduler
    keeps the same mm for prev and next.

    Suggested-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
    Cc: Chris Metcalf <cmetcalf@ezchip.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Kirill Tkhai <tkhai@yandex.ru>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    ---
    include/linux/sched/mm.h | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
    index 4a7944078cc3..8557ec664213 100644
    --- a/include/linux/sched/mm.h
    +++ b/include/linux/sched/mm.h
    @@ -362,6 +362,8 @@ enum {

    static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
    {
    + if (current->mm != mm)
    + return;
    if (likely(!(atomic_read(&mm->membarrier_state) &
    MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
    return;
    --
    2.17.1
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