Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes
From: Peter Zijlstra <peterz@infradead.org>
Date: 2018-06-14 12:41:52
Also in:
dri-devel, linux-media, lkml
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;
+}quoted hunk ↗ jump to hunk
@@ -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. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html