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

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Date:   Fri, 17 Jun 2022 07:57:53 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        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 7:43 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 think tasklist_lock
> isn't taken from softirq context, but I may be wrong. Providing a
> read_lock_unfair() will require quite a bit of work in the supporting
> infrastructure as well.
>
> Cheers,
> Longman
>