Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock() like a trylock

From: Waiman Long
Date: Wed Mar 17 2021 - 10:04:04 EST

On 3/17/21 9:31 AM, Peter Zijlstra wrote:
On Wed, Mar 17, 2021 at 02:12:41PM +0100, Peter Zijlstra wrote:
On Wed, Mar 17, 2021 at 12:38:21PM -0000, tip-bot2 for Waiman Long wrote:
+ /*
+ * Treat as trylock for ww_mutex.
+ */
+ mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, nest_lock, ip);
I'm confused... why isn't nest_lock working here?

For ww_mutex, we're supposed to have ctx->dep_map as a nest_lock, and
all lock acquisitions under a nest lock should be fine. Afaict the above
is just plain wrong.
To clarify:

mutex_lock(&A); ww_mutex_lock(&B, ctx);
ww_mutex_lock(&B, ctx); mutex_lock(&A);

should still very much be a deadlock, but your 'fix' makes it not report

Only order within the ww_ctx can be ignored, and that's exactly what
nest_lock should be doing.

I will take a deeper look into why that is the case.