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

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Date:   Fri, 17 Jun 2022 19:45:14 +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 7:42 PM Waiman Long <longman@...hat.com> wrote:
>
> On 6/17/22 11:24, Eric Dumazet wrote:
> > 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....
>
> After a second thought, I think the right way is to introduce a fair
> variant, if needed. If an arch isn't using qrwlock, the native rwlock
> implementation will be unfair. In that sense, unfair rwlock is the
> default. We will only need to change the relevant network read_lock()
> calls to use the fair variant which will still be unfair if qrwlock
> isn't used. We are not going to touch other read_lock call that don't
> care about fair or unfair.
>

Hmm... backporting this kind of invasive change to stable kernels will
be a daunting task.

Were rwlocks always unfair, and we have been lucky ?