Re: [Linaro-mm-sig] [PATCH v3 2/2] locking: Implement an algorithm choice for Wound-Wait mutexes

On Fri, Jun 15, 2018 at 02:08:27PM +0200, Thomas Hellstrom wrote:

@@ -772,6 +856,25 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter, } list_add_tail(&waiter->list, pos);

• if (__mutex_waiter_is_first(lock, waiter))

• __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
  

• /*

• * Wound-Wait: if we're blocking on a mutex owned by a younger context,
  

• * wound that such that we might proceed.
  

• */
  

• if (!is_wait_die) {

• struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
  

• /*
  

•  * See ww_mutex_set_context_fastpath(). Orders setting
  

•  * MUTEX_FLAG_WAITERS (atomic operation) vs the ww->ctx load,
  

•  * such that either we or the fastpath will wound @ww->ctx.
  

•  */
  

• smp_mb__after_atomic();
  

• __ww_mutex_wound(lock, ww_ctx, ww->ctx);
  

• }

I think we want the smp_mb__after_atomic() in the same branch as __mutex_set_flag(). So something like:

if (__mutex_waiter_is_first()) { __mutex_set_flag(); if (!is_wait_die) smp_mb__after_atomic(); }

Or possibly even without the !is_wait_die. The rules for smp_mb__*_atomic() are such that we want it unconditional after an atomic, otherwise the semantics get too fuzzy.

Alan (rightfully) complained about that a while ago when he was auditing users.