linux-kernel - Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read as read

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Date:   Fri, 3 Sep 2021 16:30:38 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] lockdep: Let lock_is_held_type() detect recursive read
 as read

On 2021-09-03 22:15:04 [+0800], Boqun Feng wrote:
> But the rwlock in PREEMPT_RT is writer unfair, isn't it? As per:
> 
> 	https://lore.kernel.org/lkml/20210815211302.957920571@linutronix.de/
> 
> also in __rwbase_read_lock():
> 
> 	/*
> 	 * Allow readers, as long as the writer has not completely
> 	 * acquired the semaphore for write.
> 	 */
> 	if (atomic_read(&rwb->readers) != WRITER_BIAS) {
> 		atomic_inc(&rwb->readers);
> 		raw_spin_unlock_irq(&rtm->wait_lock);
> 		return 0;
> 	}
> 
> that means the readers of rwlock in PREEMPT_RT are always recursive,
> right? Am I missing something subtle?

huch. You are right. I take everything backt then. I do remember it
differently but it has been like since this implementation has been
introduced…

> Regards,
> Boqun

Sebastian