linux-kernel - Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex

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Message-ID: <20200521000930.GA359643@debian-buster-darwi.lab.linutronix.de>
Date:   Thu, 21 May 2020 02:09:30 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        amd-gfx@...ts.freedesktop.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Christian König <christian.koenig@....com>,
        dri-devel@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
        Alex Deucher <alexander.deucher@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated
 wound/wait mutex

On Wed, May 20, 2020, Christian König wrote:
> Am 19.05.20 um 23:45 schrieb Ahmed S. Darwish:
> > A sequence counter write side critical section must be protected by some
> > form of locking to serialize writers. If the serialization primitive is
> > not disabling preemption implicitly, preemption has to be explicitly
> > disabled before entering the sequence counter write side critical
> > section.
> >
> > The dma-buf reservation subsystem uses plain sequence counters to manage
> > updates to reservations. Writer serialization is accomplished through a
> > wound/wait mutex.
> >
> > Acquiring a wound/wait mutex does not disable preemption, so this needs
> > to be done manually before and after the write side critical section.
> >
> > Use the newly-added seqcount_ww_mutex_t instead:
> >
> >    - It associates the ww_mutex with the sequence count, which enables
> >      lockdep to validate that the write side critical section is properly
> >      serialized.
> >
> >    - It removes the need to explicitly add preempt_disable/enable()
> >      around the write side critical section because the write_begin/end()
> >      functions for this new data type automatically do this.
> >
> > If lockdep is disabled this ww_mutex lock association is compiled out
> > and has neither storage size nor runtime overhead.
>
> Mhm, is the dma_resv object the only user of this new seqcount_ww_mutex
> variant ?
>
> If yes we are trying to get rid of this sequence counter for quite some
> time, so I would rather invest the additional time to finish this.
>

In this patch series, each extra "seqcount with associated lock" data
type costs us, exactly:

  - 1 typedef definition, seqcount_ww_mutex_t
  - 1 static initializer, SEQCNT_WW_MUTEX_ZERO()
  - 1 runtime initializer, seqcount_ww_mutex_init()

Definitions for the typedef and the 2 initializers above are
template-code one liners.

The logic which automatically disables preemption upon entering a
seqcount_ww_mutex_t write side critical section is also already shared
with seqcount_mutex_t and any future, preemptible, associated lock.

So, yes, dma-resv is the only user of seqcount_ww_mutex.

But even in that case, given the one liner template code nature of
seqcount_ww_mutex_t logic, it does not make sense to block the dma_resv
and amdgpu change until at some point in the future the sequence counter
is completely removed.

**If and when** the sequence counter gets removed, please just remove
the seqcount_ww_mutex_t data type with it. It will be extremely simple.

> Regards,
> Christian.
>

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH