Patch "locking/mcs: Fix mcs_spin_lock() ordering" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

locking/mcs: Fix mcs_spin_lock() ordering

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
locking-mcs-fix-mcs_spin_lock-ordering.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Zijlstra <[email protected]>
Date: Mon, 1 Feb 2016 15:11:28 +0100
Subject: locking/mcs: Fix mcs_spin_lock() ordering

From: Peter Zijlstra <[email protected]>

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.

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

@@ -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

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/cgroup-make-sure-a-parent-css-isn-t-freed-before-its-children.patch queue-4.4/x86-edac-sb_edac.c-repair-damage-introduced-when-fixing-channel-address.patch queue-4.4/efi-expose-non-blocking-set_variable-wrapper-to-efivars.patch queue-4.4/sched-cgroup-fix-cleanup-cgroup-teardown-init.patch queue-4.4/asm-generic-futex-re-enable-preemption-in-futex_atomic_cmpxchg_inatomic.patch queue-4.4/x86-mce-avoid-using-object-after-free-in-genpool.patch queue-4.4/futex-handle-unlock_pi-race-gracefully.patch queue-4.4/locking-mcs-fix-mcs_spin_lock-ordering.patch queue-4.4/futex-acknowledge-a-new-waiter-in-counter-before-plist.patch queue-4.4/x86-mm-xen-suppress-hugetlbfs-in-pv-guests.patch