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