linux-kernel - Re: [RFC PATCH v1 02/25] printk-rb: add prb locking functions

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Message-ID: <87y36ih8p7.fsf@linutronix.de>
Date:   Thu, 14 Feb 2019 13:10:28 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 02/25] printk-rb: add prb locking functions

On 2019-02-14, Petr Mladek <pmladek@...e.com> wrote:
>>> cpu_store looks like an implementation detail. The caller
>>> needs to remember it to handle the nesting properly.
>>> 
>>> We could achieve the same with a recursion counter hidden
>>> in struct prb_lock.
>
> The atomic operations are tricky. I feel other lost in them.
> Well, I still think that it might easier to detect nesting
> on the same CPU, see below.
>
> Also there is no need to store irq flags in per-CPU variable.
> Only the first owner of the lock need to store the flags. The others
> are spinning or nested.
>
> struct prb_cpulock {
> 	atomic_t		owner;
> 	unsigned int		flags;
> 	int			nesting; /* intialized to 0 */
> };
>
> void prb_lock(struct prb_cpulock *cpu_lock)
> {
> 	unsigned int flags;
> 	int cpu;

I added an explicit preempt_disable here:

        cpu = get_cpu();

> 	/*
> 	 * The next condition might be valid only when
> 	 * we are nested on the same CPU. It means
> 	 * the IRQs are already disabled and no
> 	 * memory barrier is needed.
> 	 */
> 	if (cpu_lock->owner == smp_processor_id()) {
> 		cpu_lock->nested++;
> 		return;
> 	}
>
> 	/* Not nested. Take the lock */
> 	local_irq_save(flags);
> 	cpu = smp_processor_id();
>
> 	for (;;) {

With fixups so it builds/runs:

                unsigned int prev_cpu = -1;

> 		if (atomic_try_cmpxchg_acquire(&cpu_lock->owner,
 					       &prev_cpu, cpu)) {
> 			cpu_lock->flags = flags;
> 			break;
> 		}
>
> 		cpu_relax();
> 	}
> }
>
> void prb_unlock(struct prb_cpulock *cpu_lock)
> {
> 	unsigned int flags;
>
> 	if (cpu_lock->nested)
> 		cpu_lock->nested--;

And the matching preempt_enable().

                goto out;

> 	}
>
> 	/* We must be the first lock owner */
> 	flags = cpu_lock->flags;
> 	atomic_set_release(&cpu_lock->owner, -1);
> 	local_irq_restore(flags);

out:
        put_cpu();

> }
>
> Or do I miss anything?

It looks great. I've run my stress tests on it and everything is running
well.

Thanks for simplifying this!

John Ogness