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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
