linux-kernel - Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu

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Message-ID: <20180220132254.GB25201@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Feb 2018 14:22:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian König <christian.koenig@....com>
Cc:     dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] locking/ww_mutex: cleanup lock->ctx usage in amdgpu

On Tue, Feb 20, 2018 at 02:08:26PM +0100, Christian König wrote:
> Am 20.02.2018 um 13:35 schrieb Peter Zijlstra:
> > > +static inline bool ww_mutex_is_owned_by(struct ww_mutex *lock,
> > > +					struct task_struct *task,
> > > +					struct ww_acquire_ctx *ctx)
> > > +{
> > > +	return likely(__mutex_owner(&lock->base) == task) &&
> > > +		READ_ONCE(lock->ctx) == ctx;
> > > +}
> > Nak on that interface, that's racy and broken by design.
> 
> Why?

If task != current you can race with a concurrent mutex_unlock().