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Date: Tue, 10 Dec 2013 17:44:37 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
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        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7]
	Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK
Message-ID: <20131210164436.GA23506@redhat.com>
References: <20131210012738.GA24317@linux.vnet.ibm.com> <1386638883-25379-1-git-send-email-paulmck@linux.vnet.ibm.com> <1386638883-25379-5-git-send-email-paulmck@linux.vnet.ibm.com>
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On 12/09, Paul E. McKenney wrote:
>
> @@ -1626,7 +1626,10 @@ for each construct.  These operations all imply certain barriers:
>       operation has completed.
>
>       Memory operations issued before the LOCK may be completed after the LOCK
> -     operation has completed.
> +     operation has completed.  An smp_mb__before_spinlock(), combined
> +     with a following LOCK, acts as an smp_wmb().  Note the "w",
> +     this is smp_wmb(), not smp_mb().

Well, but smp_mb__before_spinlock + LOCK is not wmb... But it is not
the full barrier. It should guarantee that, say,

	CONDITION = true;		// 1

	// try_to_wake_up
	smp_mb__before_spinlock();
	spin_lock(&task->pi_lock);

	if (!(p->state & state))	// 2
		return;		

can't race with with set_current_state() + check(CONDITION), this means
that 1 and 2 above must not be reordered.

But a LOAD before before spin_lock() can leak into the critical section.

Perhaps this should be clarified somehow, or perhaps it should actually
imply mb (if combined with LOCK).

Oleg