From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] locking/qrwlock: Reduce cacheline contention for rwlocks used in interrupt context
Date: Wed, 11 May 2022 15:34:59 +0200 [thread overview]
Message-ID: <20220511133459.GZ76023@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <7eff9889-56fe-503c-94ea-376054c6579b@redhat.com>
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.
next prev parent reply other threads:[~2022-05-11 13:35 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-10 19:21 [PATCH 1/2] locking/qrwlock: Change "queue rwlock" to "queued rwlock" Waiman Long
2022-05-10 19:21 ` [PATCH 2/2] locking/qrwlock: Reduce cacheline contention for rwlocks used in interrupt context Waiman Long
2022-05-11 7:20 ` Arnd Bergmann
2022-05-11 12:01 ` Waiman Long
2022-05-11 8:30 ` Peter Zijlstra
2022-05-11 12:44 ` Waiman Long
2022-05-11 13:34 ` Peter Zijlstra [this message]
2022-05-11 16:00 ` Waiman Long
2022-05-11 19:39 ` [tip: locking/core] locking/qrwlock: Change "queue rwlock" to "queued rwlock" tip-bot2 for Waiman Long
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