[tip:locking/urgent] locking/qspinlock: Ensure node->count is updated before initialising node

[tip-bot for Will Deacon]

Commit-ID:  11dc13224c975efcec96647a4768a6f1bb7a19a8
Gitweb:     https://git.kernel.org/tip/11dc13224c975efcec96647a4768a6f1bb7a19a8
Author:     Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Tue, 13 Feb 2018 13:22:57 +0000
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 13 Feb 2018 14:50:14 +0100

locking/qspinlock: Ensure node->count is updated before initialising node

When queuing on the qspinlock, the count field for the current CPU's head
node is incremented. This needn't be atomic because locking in e.g. IRQ
context is balanced and so an IRQ will return with node->count as it
found it.

However, the compiler could in theory reorder the initialisation of
node[idx] before the increment of the head node->count, causing an
IRQ to overwrite the initialised node and potentially corrupt the lock
state.

Avoid the potential for this harmful compiler reordering by placing a
barrier() between the increment of the head node->count and the subsequent
node initialisation.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1518528177-19169-3-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/locking/qspinlock.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 348c8ce..d880296 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -379,6 +379,14 @@ queue:
 	tail = encode_tail(smp_processor_id(), idx);
 
 	node += idx;
+
+	/*
+	 * Ensure that we increment the head node->count before initialising
+	 * the actual node. If the compiler is kind enough to reorder these
+	 * stores, then an IRQ could overwrite our assignments.
+	 */
+	barrier();
+
 	node->locked = 0;
 	node->next = NULL;
 	pv_init_node(node);