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authorWill Deacon <will.deacon@arm.com>2015-12-11 17:46:41 +0000
committerLinus Torvalds <torvalds@linux-foundation.org>2015-12-17 11:40:29 -0800
commitb4b29f94856ad68329132c2306e9a114920643e3 (patch)
tree6109868576fbdc7bbbe6cddba7ddec62ca920b67
parentd7637d01bef21848d156a219626b6541ccc0a734 (diff)
locking/osq: Fix ordering of node initialisation in osq_lock
The Cavium guys reported a soft lockup on their arm64 machine, caused by commit c55a6ffa6285 ("locking/osq: Relax atomic semantics"): mutex_optimistic_spin+0x9c/0x1d0 __mutex_lock_slowpath+0x44/0x158 mutex_lock+0x54/0x58 kernfs_iop_permission+0x38/0x70 __inode_permission+0x88/0xd8 inode_permission+0x30/0x6c link_path_walk+0x68/0x4d4 path_openat+0xb4/0x2bc do_filp_open+0x74/0xd0 do_sys_open+0x14c/0x228 SyS_openat+0x3c/0x48 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@caviumnetworks.com> Reported-by: Andrew Pinski <andrew.pinski@caviumnetworks.com> Tested-by: Andrew Pinski <andrew.pinski@caviumnetworks.com> Acked-by: Davidlohr Bueso <dave@stgolabs.net> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: http://lkml.kernel.org/r/1449856001-21177-1-git-send-email-will.deacon@arm.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--kernel/locking/osq_lock.c8
1 files changed, 5 insertions, 3 deletions
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index d092a0c9c2d4..05a37857ab55 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -93,10 +93,12 @@ bool osq_lock(struct optimistic_spin_queue *lock)
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;