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SubjectRe: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
On Wed, Jan 22, 2020 at 11:38:57AM -0500, Qian Cai wrote:
> KCSAN complains,
>
> write (marked) to 0xffff941ca3b3be00 of 8 bytes by task 670 on cpu 6:
> osq_lock+0x24c/0x340
> __mutex_lock+0x277/0xd20
> mutex_lock_nested+0x31/0x40
> memcg_create_kmem_cache+0x2e/0x190
> memcg_kmem_cache_create_func+0x40/0x80
> process_one_work+0x54c/0xbe0
> worker_thread+0x80/0x650
> kthread+0x1e0/0x200
> ret_from_fork+0x27/0x50
>
> read to 0xffff941ca3b3be00 of 8 bytes by task 703 on cpu 44:
> osq_lock+0x18e/0x340
> __mutex_lock+0x277/0xd20
> mutex_lock_nested+0x31/0x40
> memcg_create_kmem_cache+0x2e/0x190
> memcg_kmem_cache_create_func+0x40/0x80
> process_one_work+0x54c/0xbe0
> worker_thread+0x80/0x650
> kthread+0x1e0/0x200
> ret_from_fork+0x27/0x50
>
> which points to those lines in osq_wait_next(),
>
> next = xchg(&node->next, NULL);
> if (next)
> break;
>
> Since only the read is outside of critical sections, fixed it by adding
> a READ_ONCE().
>
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> kernel/locking/osq_lock.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 6ef600aa0f47..8f565165019a 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -77,7 +77,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
> */
> if (node->next) {
> next = xchg(&node->next, NULL);
> - if (next)
> + if (READ_ONCE(next))
> break;
> }

I don't understand this; 'next' is a local variable.

Not keen on the onslaught of random "add a READ_ONCE() to shut the
sanitiser up" patches we're going to get from kcsan :(

Will

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