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

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Date:   Fri, 28 Aug 2020 10:57:50 +0200
From:   peterz@...radead.org
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
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 03:07:09AM +0200, Ahmed S. Darwish wrote:
> +/*
> + * 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))

Hurph, so basically you want to make __seqcount_lock_preemptible()
return true PREEMPT_RT ? Should we then not muck about with the propery
instead of this?

ISTR I had something like the below, would that not be the same but much
clearer ?

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 300cbf312546..3b5ad026ddfb 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -211,11 +211,13 @@ static inline void __seqcount_assert(seqcount_t *s)
 	lockdep_assert_preemption_disabled();
 }
 
-SEQCOUNT_LOCKTYPE(raw_spinlock_t,	raw_spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(spinlock_t,		spinlock,	false,	s->lock)
-SEQCOUNT_LOCKTYPE(rwlock_t,		rwlock,		false,	s->lock)
-SEQCOUNT_LOCKTYPE(struct mutex,		mutex,		true,	s->lock)
-SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,	&s->lock->base)
+#define __PREEMPT_RT	IS_BUILTIN(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKTYPE(raw_spinlock_t,	raw_spinlock,	false,		s->lock)
+SEQCOUNT_LOCKTYPE(spinlock_t,		spinlock,	__PREEMPT_RT,	s->lock)
+SEQCOUNT_LOCKTYPE(rwlock_t,		rwlock,		__PREEMPT_RT,	s->lock)
+SEQCOUNT_LOCKTYPE(struct mutex,		mutex,		true,		s->lock)
+SEQCOUNT_LOCKTYPE(struct ww_mutex,	ww_mutex,	true,		&s->lock->base)
 
 /*
  * SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t