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.
