[PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex
Ahmed S. Darwish
a.darwish at linutronix.de
Thu May 21 00:09:30 UTC 2020
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.
Ahmed S. Darwish
More information about the amd-gfx