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

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Message-ID: <20200828085938.GS1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Aug 2020 10:59:38 +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:
> +#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.

> +/*
> + * 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.