Date: Thu, 23 Jan 2014 11:07:51 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "Aswin Chandramouleeswaran\"" <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH v10 1/4] qrwlock: A queue read/write lock implementation
Message-ID: <20140123100751.GS30183@twins.programming.kicks-ass.net>
References: <1390426438-31252-1-git-send-email-Waiman.Long@hp.com>
 <1390426438-31252-2-git-send-email-Waiman.Long@hp.com>
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On Wed, Jan 22, 2014 at 04:33:55PM -0500, Waiman Long wrote:
> +/**
> + * queue_read_unlock - release read lock of a queue rwlock
> + * @lock : Pointer to queue rwlock structure
> + */
> +static inline void queue_read_unlock(struct qrwlock *lock)
> +{
> +	/*
> +	 * Atomically decrement the reader count
> +	 */
> +	atomic_sub(_QR_BIAS, &lock->cnts.rwa);
> +}
> +
> +/**
> + * queue_write_unlock - release write lock of a queue rwlock
> + * @lock : Pointer to queue rwlock structure
> + */
> +static inline void queue_write_unlock(struct qrwlock *lock)
> +{
> +	/*
> +	 * If the writer field is atomic, it can be cleared directly.
> +	 * Otherwise, an atomic subtraction will be used to clear it.
> +	 */
> +	if (__native_word(lock->cnts.writer))
> +		smp_store_release(&lock->cnts.writer, 0);
> +	else
> +		atomic_sub(_QW_LOCKED, &lock->cnts.rwa);
> +}

Both these unlocks miss a barrier; atomic_sub() doesn't imply any
barrier what so ever.

The smp_store_release() does, but the other two are invalid release ops
in generic.
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