[PATCH] locking/osq: Fix ordering of node initialisation in osq_lock

[Peter Zijlstra]

Hi Linus,

Please consider this patch for 4.4.

---
Subject: locking/osq: Fix ordering of node initialisation in osq_lock
From: Will Deacon <will.deacon@xxxxxxx>
Date: Fri, 11 Dec 2015 17:46:41 +0000

The Cavium guys reported a soft lockup on their arm64 machine, caused
by c55a6ffa6285 ("locking/osq: Relax atomic semantics"):

[   68.909948] [<fffffe00000da03c>] mutex_optimistic_spin+0x9c/0x1d0
[   68.909951] [<fffffe00006fe4b8>] __mutex_lock_slowpath+0x44/0x158
[   68.909953] [<fffffe00006fe620>] mutex_lock+0x54/0x58
[   68.909956] [<fffffe0000265efc>] kernfs_iop_permission+0x38/0x70
[   68.909959] [<fffffe00001fbf50>] __inode_permission+0x88/0xd8
[   68.909961] [<fffffe00001fbfd0>] inode_permission+0x30/0x6c
[   68.909964] [<fffffe00001fe26c>] link_path_walk+0x68/0x4d4
[   68.909966] [<fffffe00001ffa14>] path_openat+0xb4/0x2bc
[   68.909968] [<fffffe000020123c>] do_filp_open+0x74/0xd0
[   68.909971] [<fffffe00001f13e4>] do_sys_open+0x14c/0x228
[   68.909973] [<fffffe00001f1544>] SyS_openat+0x3c/0x48
[   68.909976] [<fffffe00000851f0>] el0_svc_naked+0x24/0x28

This is because in osq_lock we initialise the node for the current CPU:

	node->locked = 0;
	node->next = NULL;
	node->cpu = curr;

and then publish the current CPU in the lock tail:

	old = atomic_xchg_acquire(&lock->tail, curr);

Once the update to lock->tail is visible to another CPU, the node is
then live and can be both read and updated by concurrent lockers.

Unfortunately, the ACQUIRE semantics of the xchg operation mean that
there is no guarantee the contents of the node will be visible before
lock tail is updated. This can lead to lock corruption when, for example,
a concurrent locker races to set the next field.

Fixes: c55a6ffa6285 ("locking/osq: Relax atomic semantics"):
Reported-by: David Daney <ddaney@xxxxxxxxxxxxxxxxxx>
Reported-by: Andrew Pinski <andrew.pinski@xxxxxxxxxxxxxxxxxx>
Tested-by: Andrew Pinski <andrew.pinski@xxxxxxxxxxxxxxxxxx>
Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1449856001-21177-1-git-send-email-will.deacon@xxxxxxx
---
 kernel/locking/osq_lock.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -93,10 +93,12 @@ bool osq_lock(struct optimistic_spin_que
 	node->cpu = curr;
 
 	/*
-	 * ACQUIRE semantics, pairs with corresponding RELEASE
-	 * in unlock() uncontended, or fastpath.
+	 * We need both ACQUIRE (pairs with corresponding RELEASE in
+	 * unlock() uncontended, or fastpath) and RELEASE (to publish
+	 * the node fields we just initialised) semantics when updating
+	 * the lock tail.
 	 */
-	old = atomic_xchg_acquire(&lock->tail, curr);
+	old = atomic_xchg(&lock->tail, curr);
 	if (old == OSQ_UNLOCKED_VAL)
 		return true;
 
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