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    SubjectRe: [PATCH locking/Documentation 1/2] Add note of release-acquire store vulnerability
    On Thu, Sep 29, 2016 at 06:17:38PM +0200, Peter Zijlstra wrote:
    > On Thu, Sep 29, 2016 at 05:03:08PM +0100, Will Deacon wrote:
    > > On Thu, Sep 29, 2016 at 05:58:17PM +0200, Peter Zijlstra wrote:
    > > > 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@rowland.harvard.edu>
    > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.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.
    > >
    > > If I've groked it correctly, it's for cases like:
    > >
    > >
    > > PO:
    > > Wx=1
    > > WyRel=1
    > >
    > > P1:
    > > Wy=2
    > >
    > > P2:
    > > RyAcq=2
    > > Rx=0
    > >
    > > Final value of y is 2.
    > >
    > >
    > > This is permitted on arm64. If you make P1's store a store-release, then
    > > it's forbidden, but I suspect that's not generally true of the kernel
    > > memory model.
    >
    > Right, I think that on PowerPC, even if P1 does store-release you can
    > still get this, since the two stores conflict one can loose out, and the
    > lwsync associated with the loosing store gets removed along with it.
    >
    > So yes, I think this needs more clarification.

    Whether the store is loose or not, yes, putting an lwsync as the first
    instruction in a given task has no effect. ;-)

    Thanx, Paul

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