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    Subject[PATCH 4.4 116/163] locking/mcs: Fix mcs_spin_lock() ordering
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Peter Zijlstra <peterz@infradead.org>

    commit 920c720aa5aa3900a7f1689228fdfc2580a91e7e upstream.

    Similar to commit b4b29f94856a ("locking/osq: Fix ordering of node
    initialisation in osq_lock") the use of xchg_acquire() is
    fundamentally broken with MCS like constructs.

    Furthermore, it turns out we rely on the global transitivity of this
    operation because the unlock path observes the pointer with a
    READ_ONCE(), not an smp_load_acquire().

    This is non-critical because the MCS code isn't actually used and
    mostly serves as documentation, a stepping stone to the more complex
    things we've build on top of the idea.

    Reported-by: Andrea Parri <parri.andrea@gmail.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Will Deacon <will.deacon@arm.com>
    Fixes: 3552a07a9c4a ("locking/mcs: Use acquire/release semantics")
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/locking/mcs_spinlock.h | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- a/kernel/locking/mcs_spinlock.h
    +++ b/kernel/locking/mcs_spinlock.h
    @@ -67,7 +67,13 @@ void mcs_spin_lock(struct mcs_spinlock *
    node->locked = 0;
    node->next = NULL;

    - prev = xchg_acquire(lock, node);
    + /*
    + * We rely on the full barrier with global transitivity implied by the
    + * below xchg() to order the initialization stores above against any
    + * observation of @node. And to provide the ACQUIRE ordering associated
    + * with a LOCK primitive.
    + */
    + prev = xchg(lock, node);
    if (likely(prev == NULL)) {
    /*
    * Lock acquired, don't need to set node->locked to 1. Threads

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