linux-kernel - Re: [PATCH v1 4/5] seqlock: seqcount_LOCKTYPE_t: Introduce PREEMPT

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Date:   Fri, 28 Aug 2020 16:36:23 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     peterz@...radead.org
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 4/5] seqlock: seqcount_LOCKTYPE_t: Introduce
 PREEMPT_RT support

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@...radead.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