On 06/14/2018 02:48 PM, Thomas Hellstrom wrote:
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.
Or actually it was intended to be true, but FLAG_WAITERS is set too
late. It needs to be moved to just after we actually add the waiter to
the list.
Then the hunk that replaces a FLAG_WAITERS read with a lockless
list_empty() can also be ditched.
/Thomas
So the wake_up_process() is in fact safe.
Let me put that in a comment.
Thanks,
Thomas
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