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

[Christian KÃnig]

Am 21.05.20 um 02:09 schrieb Ahmed S. Darwish:
> On Wed, May 20, 2020, Christian KÃnig 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.

In this case I'm perfectly fine with this.

> 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.

Completely agree, just wanted to prevent that we now add a lot of code 
which gets removed again ~3 month from now.

Regards,
Christian.

> > Regards,
> > Christian.
> Thanks,
>
> --
> Ahmed S. Darwish
> Linutronix GmbH