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

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Date:   Fri, 17 Jun 2022 10:43:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     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>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH] locking/rwlocks: do not starve writers

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. 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