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Date:   Tue, 23 Apr 2019 21:34:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
 negative
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On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote:
> On 4/23/19 12:27 PM, Linus Torvalds wrote:
> > On Tue, Apr 23, 2019 at 7:17 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >> I'm not aware of an architecture where disabling interrupts is faster
> >> than disabling preemption.
> > I don't thin kit ever is, but I'd worry a bit about the
> > preempt_enable() just because it also checks if need_resched() is true
> > when re-enabling preemption.
> >
> > So doing preempt_enable() as part of rwsem_read_trylock() might cause
> > us to schedule in *exactly* the wrong place,
> 
> You are right on that. However, there is a variant called
> preempt_enable_no_resched() that doesn't have this side effect. So I am
> going to use that one instead.

Only if the very next line is schedule(). Otherwise you're very much not
going to use that function.