Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

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Hi, Peter,

On 06/14/2018 02:41 PM, Peter Zijlstra wrote:
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.

If __ww_mutex_wound() is called from ww_mutex_set_context_fastpath() owner is the current process so we can never see !owner. However if __ww_mutex_wound() is called from __ww_mutex_add_waiter() then the above is true.


So the wake_up_process() is in fact safe.

Let me put that in a comment.


Thanks,

Thomas


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