linux-kernel - Re: [PATCH locking/Documentation 1/2] Add note of release-acquire store vulnerability

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Message-ID: <20160929155817.GB5016@twins.programming.kicks-ass.net>
Date:   Thu, 29 Sep 2016 17:58:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        dhowells@...hat.com, will.deacon@....com, stern@...land.harvard.edu
Subject: Re: [PATCH locking/Documentation 1/2] Add note of release-acquire
 store vulnerability

On Thu, Sep 29, 2016 at 08:54:01AM -0700, Paul E. McKenney wrote:
> If two processes are related by a RELEASE+ACQUIRE pair, ordering can be
> broken if a third process overwrites the value written by the RELEASE
> operation before the ACQUIRE operation has a chance of reading it.
> This commit therefore updates the documentation to call this vulnerability
> out explicitly.
> 
> Reported-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> +     However, please note that a chain of RELEASE+ACQUIRE pairs may be
> +     broken by a store by another thread that overwrites the RELEASE
> +     operation's store before the ACQUIRE operation's read.

This is the powerpc lwsync quirk, right? Where the barrier disappears
when it looses the store.

Or is there more to it? Its not entirely clear from the Changelog, which
I feel should describe the reason for the behaviour.