linux-kernel - Re: [PATCH] locking/qrwlock: queued_write_lock

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Message-ID: <d8b2bc8a-3788-6cba-ffa9-2985d7a278d4@redhat.com>
Date:   Mon, 26 Apr 2021 10:54:41 -0400
From:   Waiman Long <llong@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Ali Saidi <alisaidi@...zon.com>,
        Steve Capper <steve.capper@....com>
Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup

On 4/26/21 4:08 AM, Peter Zijlstra wrote:
> On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote:
>
>>   void queued_write_lock_slowpath(struct qrwlock *lock)
>>   {
>> -	int cnts;
>> +	int cnts = 0;
>>   
>>   	/* Put the writer into the wait queue */
>>   	arch_spin_lock(&lock->wait_lock);
>>   
>>   	/* Try to acquire the lock directly if no reader is present */
>>   	if (!atomic_read(&lock->cnts) &&
>> -	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
>> +	    atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
>>   		goto unlock;
> Would not something like:
>
> 	if (!(cnts = atomic_read(&lock->cnts)) &&
> 	    atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)
> 		goto unlock;
>
> Be clearer?
That works for me too. It is equivalent anyway.
>
>>   
>> -	/* Set the waiting flag to notify readers that a writer is pending */
>> -	atomic_add(_QW_WAITING, &lock->cnts);
>> +	/*
>> +	 * Set the waiting flag to notify readers that a writer is pending
>> +	 *
>> +	 * As only one writer who is the wait_lock owner can set the waiting
>> +	 * flag which will be cleared later on when acquiring the write lock,
>> +	 * we can easily replace atomic_or() by an atomic_add() if there is
>> +	 * an architecture where an atomic_add() performs better than an
>> +	 * atomic_or().
> That might be a little overboard on the comment, but sure :-) I don't
> think there's any arch that doesn't have atomic_or(), like I wrote
> elsewhere, the one that's often an issue is atomic_fetch_or().
>
I was not sure as I didn't look at other archs that hadn't used qrwlock 
yet. Given what you said, I will remove the comment.

Cheers,
Longman