linux-kernel - Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned

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Date:   Tue, 20 Feb 2018 15:54:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian König <christian.koenig@....com>
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dev@...ankhorst.nl,
        chris@...is-wilson.co.uk
Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function
 v3

On Tue, Feb 20, 2018 at 03:34:07PM +0100, Christian König wrote:
> > OK, but neither case would in fact need the !ctx case right? That's just
> > there for completeness sake?
> 
> Unfortunately not. TTM uses trylock to lock BOs which are about to be
> evicted to make room for all the BOs locked with a ctx.
> 
> I need to be able to distinct between the BOs which are trylocked and those
> which are locked with a ctx.
> 
> Writing this I actually noticed the current version is buggy, cause even
> when we check the mutex owner we still need to make sure that the ctx in the
> lock is NULL.

Hurm... I can't remember why trylocks behave like that, and it seems
rather unfortunate / inconsistent.

Chris, Maarten, do either one of you remember?

I'm thinking that if we do acquire the trylock, the thing should join
the ctx such that a subsequent contending mutex_lock() can ww right.