linux-kernel - Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative

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Message-ID: <88ba7be0-9ec5-941e-1b3f-80fbe05fe3a0@redhat.com>
Date:   Tue, 23 Apr 2019 15:41:32 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
 negative

On 4/23/19 3:34 PM, Peter Zijlstra wrote:
> 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@...radead.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.

May I know the reason why. I saw a number of instances of
preempt_enable_no_resched() without right next a schedule().

Cheers,
Longman