linux-kernel - Re: [PATCH] locking/rwlocks: do not starve writers

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Message-ID: <CANn89iJLWJMmNrLYQ0EU7_0Wri6c3Kn9vYMOiWu1Ds8Af2KOnw@mail.gmail.com>
Date:   Fri, 17 Jun 2022 17:24:58 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, Roman Penyaev <rpenyaev@...e.de>
Subject: Re: [PATCH] locking/rwlocks: do not starve writers

On Fri, Jun 17, 2022 at 5:00 PM Waiman Long <longman@...hat.com> wrote:
>
> On 6/17/22 10:57, Shakeel Butt wrote:
> > On Fri, Jun 17, 2022 at 7:43 AM Waiman Long <longman@...hat.com> wrote:
> >> On 6/17/22 08:07, Peter Zijlstra wrote:
> >>> On Fri, Jun 17, 2022 at 02:10:39AM -0700, Eric Dumazet wrote:
> >>>> --- a/kernel/locking/qrwlock.c
> >>>> +++ b/kernel/locking/qrwlock.c
> >>>> @@ -23,16 +23,6 @@ void queued_read_lock_slowpath(struct qrwlock *lock)
> >>>>       /*
> >>>>        * Readers come here when they cannot get the lock without waiting
> >>>>        */
> >>>> -    if (unlikely(in_interrupt())) {
> >>>> -            /*
> >>>> -             * Readers in interrupt context will get the lock immediately
> >>>> -             * if the writer is just waiting (not holding the lock yet),
> >>>> -             * so spin with ACQUIRE semantics until the lock is available
> >>>> -             * without waiting in the queue.
> >>>> -             */
> >>>> -            atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
> >>>> -            return;
> >>>> -    }
> >>>>       atomic_sub(_QR_BIAS, &lock->cnts);
> >>>>
> >>>>       trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ);
> >>> This is known to break tasklist_lock.
> >>>
> >> We certainly can't break the current usage of tasklist_lock.
> >>
> >> I am aware of this problem with networking code and is thinking about
> >> either relaxing the check to exclude softirq or provide a
> >> read_lock_unfair() variant for networking use.
> > read_lock_unfair() for networking use or tasklist_lock use?
>
> I mean to say read_lock_fair(), but it could also be the other way
> around. Thanks for spotting that.
>

If only tasklist_lock is problematic and needs the unfair variant,
then changing a few read_lock() for tasklist_lock will be less
invasive than ~1000 read_lock() elsewhere....