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

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Date:   Tue, 23 Apr 2019 15:12:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 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.

> So if we play preemption games, I wonder if we should make them more
> explicit than hiding them in that helper function, because
> particularly for the slow path case, I think we'd be much better off
> just avoiding the busy-loop in the slow path, rather than first
> scheduling due to preempt_enable(), and then starting to look at the
> slow path onlly afterwards.
>
> IOW, I get the feeling that the preemption-off area might be better
> off being potentially much bigger, and covering the whole (or a large
> portion) of the semaphore operation, rather than just the
> rwsem_read_trylock() fastpath.
>
> Hmm?

That is true in general, but doing preempt_disable/enable across
function boundary is ugly and prone to further problems down the road.

Cheers,
Longman

>                    Linus