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    Subject[PATCH v3 2/9] task_work: Replace spin_unlock_wait() with lock/unlock pair
    Date
    From: Oleg Nesterov <oleg@redhat.com>

    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
    task_work_run() with a spin_lock_irq() and a spin_unlock_irq() aruond
    the cmpxchg() dequeue loop. This should be safe from a performance
    perspective because ->pi_lock is local to the task and because calls to
    the other side of the race, task_work_cancel(), should be rare.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/task_work.c | 8 ++------
    1 file changed, 2 insertions(+), 6 deletions(-)

    diff --git a/kernel/task_work.c b/kernel/task_work.c
    index d513051fcca2..836a72a66fba 100644
    --- a/kernel/task_work.c
    +++ b/kernel/task_work.c
    @@ -96,20 +96,16 @@ void task_work_run(void)
    * work->func() can do task_work_add(), do not set
    * work_exited unless the list is empty.
    */
    + raw_spin_lock_irq(&task->pi_lock);
    do {
    work = READ_ONCE(task->task_works);
    head = !work && (task->flags & PF_EXITING) ?
    &work_exited : NULL;
    } while (cmpxchg(&task->task_works, work, head) != work);
    + raw_spin_unlock_irq(&task->pi_lock);

    if (!work)
    break;
    - /*
    - * Synchronize with task_work_cancel(). It can't remove
    - * the first entry == work, cmpxchg(task_works) should
    - * fail, but it can play with *work and other entries.
    - */
    - raw_spin_unlock_wait(&task->pi_lock);

    do {
    next = work->next;
    --
    2.5.2
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