Re: [PATCH v1 4/5] seqlock: seqcount_LOCKTYPE_t: Introduce PREEMPT_RT support

Ahmed S. Darwish
2020-08-28T14:36:34Z

On Fri, Aug 28, 2020 at 11:31:32AM +0200, Ahmed S. Darwish wrote:
> On Fri, Aug 28, 2020 at 10:59:38AM +0200, peterz@infradead.org wrote:
> > On Fri, Aug 28, 2020 at 03:07:09AM +0200, Ahmed S. Darwish wrote:
> > > +#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
> > > +
> > > +SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
> > > +SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))
> >
> > Ooh, reading is hard, but ^^^^ you already have that.
> >
>
> Haha, I was just answering the other mail :)
>
> > > +/*
> > > + * Automatically disable preemption for seqcount_LOCKTYPE_t writers, if the
> > > + * associated lock does not implicitly disable preemption.
> > > + *
> > > + * Don't do it for PREEMPT_RT. Check __SEQ_LOCK().
> > > + */
> > > +#define __seq_enforce_preemption_protection(s) \
> > > + (!IS_ENABLED(CONFIG_PREEMPT_RT) && __seqcount_lock_preemptible(s))
> >
> > Then what is this doing ?!? I'm confused now.
>
> There were a number of call sites (at DRM mainly) that protected their
> seqcount_t write sections with mutex and ww_mutex. So, they manually
> disabled preemption before entering the seqcount_t write sections.
>
> Unfortunately these write sections were big, and spinlocks were also
> acquired inside them. This was all very bad for RT...
>
> So the idea was to relieve call sites from the responsibility of
> disabling/enabling preemption upon entering a seqcount_LOCKNAME_t write
> section, and let seqlock.h automatically handle it if LOCKNAME_t is
> preemptible (the macro's condition, second part).
>
> Having seqlock.h control the preempt disable/enable allows us to never
> disable preemption for RT, which is exactly what is needed since we
> already handle any possible livelock through the mechanisms described at
> __SEQ_LOCK (the macro's condition test, first part).
>

So to summarize, __seqcount_lock_preemptible() has two use cases:

1. For !PREEMPT_RT, to automatically disable preemption on the write
side when the seqcount associated lock is preemptible.

2. For PREEMPT_RT, to lock/unlock the seqcount associated lock on
the read side if it is RT-preemptible (sleeping lock).

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH