Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 26 Apr 2021 10:08:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, Borislav Petkov <bp@suse.de>,
        Ali Saidi <alisaidi@amazon.com>,
        Steve Capper <steve.capper@arm.com>
Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup
Message-ID: <YIZ0+hNh0SMQoOkh@hirez.programming.kicks-ass.net>
References: <20210425200637.31298-1-longman@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210425200637.31298-1-longman@redhat.com>
Precedence: bulk

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?

>  
> -	/* 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().

> +	 */
> +	atomic_or(_QW_WAITING, &lock->cnts);