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

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Message-ID: <20180220152158.GH25201@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Feb 2018 16:21:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     christian.koenig@....com
Cc:     dev@...ankhorst.nl, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function
 v3

On Tue, Feb 20, 2018 at 04:05:49PM +0100, Christian König wrote:
> Am 20.02.2018 um 15:54 schrieb Peter Zijlstra:
> > 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.
> 
> Actually for me that is rather fortunate, cause I need to distinct between
> the locks acquired through trylock and lock.

I suppose that would always be possible using:
ww_mutex_trylock(.ctx=NULL), and it could be that there simply weren't
any immediate uses for a !NULL trylock and it was thus not implemented.

But that is all very long ago..