linux-kernel - Re: [PATCH 2/2] locking/qrwlock: Reduce cacheline contention for rwlocks used in interrupt context

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Message-ID: <b64b39ab-58a0-8046-026a-8d635f3f762b@redhat.com>
Date:   Wed, 11 May 2022 12:00:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] locking/qrwlock: Reduce cacheline contention for
 rwlocks used in interrupt context

On 5/11/22 09:34, Peter Zijlstra wrote:
> On Wed, May 11, 2022 at 08:44:55AM -0400, Waiman Long wrote:
>
>>> I'm confused; prior to this change:
>>>
>>> 	CPU0			CPU1
>>>
>>> 	write_lock_irq(&l)
>>> 				read_lock(&l)
>>> 				<INRQ>
>>> 				  read_lock(&l)
>>> 				  ...
>>>
>>> was not deadlock, but now it would AFAICT.
>> Oh you are right. I missed that scenario in my analysis. My bad.
> No worries; I suppose we can also still do something like:
>
> void queued_read_lock_slowpath(struct qrwlock *lock, int cnts)
> {
> 	/*
> 	 * the big comment
> 	 */
> 	if (unlikely(in_interrupt())) {
> 		/*
> 		 * If not write-locked, insta-grant the reader
> 		 */
> 		if (!(cnts & _QW_LOCKED))
> 			return;
>
> 		/*
> 		 * otherwise, wait for the writer to go away.
> 		 */
> 		atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> 		return;
> 	}
>
> 	...
> }
>
> Which saves one load in some cases... not sure it's worth it though.

Yes, it is a micro-optimization that can be done. The gain, if any, 
should be minor though.

Cheers,
Longman