locking/mcs: Fix mcs_spin_lock() ordering

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>

diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index 5b9102a47ea5209dd887b6dfbca16215b2bad922..c835270f0c2f93c24f623e9ea2ba2e94eebb006a 100644 (file)
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -67,7 +67,13 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
        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