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    SubjectRe: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes
    On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
    > +static bool __ww_mutex_wound(struct mutex *lock,
    > + struct ww_acquire_ctx *ww_ctx,
    > + struct ww_acquire_ctx *hold_ctx)
    > +{
    > + struct task_struct *owner = __mutex_owner(lock);
    > +
    > + lockdep_assert_held(&lock->wait_lock);
    > +
    > + if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
    > + ww_ctx->acquired > 0) {
    > + hold_ctx->wounded = 1;
    > +
    > + /*
    > + * wake_up_process() paired with set_current_state() inserts
    > + * sufficient barriers to make sure @owner either sees it's
    > + * wounded or has a wakeup pending to re-read the wounded
    > + * state.
    > + *
    > + * The value of hold_ctx->wounded in
    > + * __ww_mutex_lock_check_stamp();
    > + */
    > + if (owner != current)
    > + wake_up_process(owner);
    > +
    > + return true;
    > + }
    > +
    > + return false;
    > +}

    > @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
    > * and keep spinning, or it will acquire wait_lock, add itself
    > * to waiter list and sleep.
    > */
    > - smp_mb(); /* ^^^ */
    > + smp_mb(); /* See comments above and below. */
    >
    > /*
    > - * Check if lock is contended, if not there is nobody to wake up
    > + * Check if lock is contended, if not there is nobody to wake up.
    > + * We can use list_empty() unlocked here since it only compares a
    > + * list_head field pointer to the address of the list head
    > + * itself, similarly to how list_empty() can be considered RCU-safe.
    > + * The memory barrier above pairs with the memory barrier in
    > + * __ww_mutex_add_waiter and makes sure lock->ctx is visible before
    > + * we check for waiters.
    > */
    > - if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
    > + if (likely(list_empty(&lock->base.wait_list)))
    > return;
    >

    OK, so what happens is that if we see !empty list, we take wait_lock,
    if we end up in __ww_mutex_wound() we must really have !empty wait-list.

    It can however still see !owner because __mutex_unlock_slowpath() can
    clear the owner field. But if owner is set, it must stay valid because
    FLAG_WAITERS and we're holding wait_lock.

    So the wake_up_process() is in fact safe.

    Let me put that in a comment.

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