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Message-ID: <2dd754f9-3a79-ed17-e423-6b411c3afb69@redhat.com>
Date:   Fri, 17 Jun 2022 10:43:16 -0400
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Subject: Re: [PATCH] locking/rwlocks: do not starve writers
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To:     Peter Zijlstra <peterz@infradead.org>,
        Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will@kernel.org>,
        Roman Penyaev <rpenyaev@suse.de>,
        Shakeel Butt <shakeelb@google.com>
References: <20220617091039.2257083-1-eric.dumazet@gmail.com>
 <YqxufxqsnHjVfQOs@worktop.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YqxufxqsnHjVfQOs@worktop.programming.kicks-ass.net>
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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