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

On 06/14/2018 01:36 PM, Peter Zijlstra wrote:

On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:

...

__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) { struct mutex_waiter *cur;

• unsigned int is_wait_die = ww_ctx->ww_class->is_wait_die;

lockdep_assert_held(&lock->wait_lock); @@ -310,13 +348,14 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) if (!cur->ww_ctx) continue;

• if (cur->ww_ctx->acquired > 0 &&
  

• if (is_wait_die && cur->ww_ctx->acquired > 0 &&
  

  __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) { debug_mutex_wake_waiter(lock, cur); wake_up_process(cur->task); }

• break;
  

• if (is_wait_die || __ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
  

• 	break;
  

  } }

I ended up with:

static void __sched __ww_mutex_check_waiters(struct mutex *lock, struct ww_acquire_ctx *ww_ctx) { bool is_wait_die = ww_ctx->ww_class->is_wait_die; struct mutex_waiter *cur;

lockdep_assert_held(&lock->wait_lock);

list_for_each_entry(cur, &lock->wait_list, list) { if (!cur->ww_ctx) continue;

if (is_wait_die) {
	/*
	 * Because __ww_mutex_add_waiter() and
	 * __ww_mutex_check_stamp() wake any but the earliest
	 * context, this can only affect the first waiter (with
	 * a context).
	 */
	if (cur->ww_ctx->acquired > 0 &&
	    __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
		debug_mutex_wake_waiter(lock, cur);
		wake_up_process(cur->task);
	}

	break;
}

if (__ww_mutex_wound(lock, cur->ww_ctx, ww_ctx))
	break;

} }

Looks OK to me.

Currently you don't allow mixing WD and WW contexts (which is not immediately obvious from the above code), and the above hard relies on that. Are there sensible use cases for mixing them? IOW will your current restriction stand without hassle?

Contexts _must_ agree on the algorithm used to resolve deadlocks. With Wait-Die, for example, older transactions will wait if a lock is held by a younger transaction and with Wound-Wait, younger transactions will wait if a lock is held by an older transaction so there is no way of mixing them.

Thanks,

/Thomas