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

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Message-ID: <2730b855-8f99-5a9e-707e-697d3bd9811d@redhat.com>
Date:   Fri, 17 Jun 2022 11:00:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Shakeel Butt <shakeelb@...gle.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 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.

Cheers,
Longman