Re: [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner

Waiman Long
2020-01-14T18:17:57Z

On 1/14/20 11:12 AM, Christoph Hellwig wrote:
> The rwsem code overloads the owner field with either a task struct or
> negative magic numbers. Add a quick hack to catch these negative
> values early on. Without this spinning on a writer that replaced the
> owner with RWSEM_OWNER_UNKNOWN, rwsem_spin_on_owner can crash while
> deferencing the task_struct ->on_cpu field of a -8 value.
>
> XXX: This might be a bit of a hack as the code otherwise doesn't use
> the ERR_PTR family macros, better suggestions welcome.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> kernel/locking/rwsem.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 44e68761f432..6adc719a30a1 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -725,6 +725,8 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
> state = rwsem_owner_state(owner, flags, nonspinnable);
> if (state != OWNER_WRITER)
> return state;
> + if (IS_ERR(owner))
> + return state;
>
> rcu_read_lock();
> for (;;) {

The owner field is just a pointer to the task structure with the lower 3
bits served as flag bits. Setting owner to RWSEM_OWNER_UNKNOWN (-2) will
stop optimistic spinning. So under what condition did the crash happen?

Anyway, PeterZ is working on revising the percpu-rwsem implementation to
more gracefully handle the frozen case. At the end, there will not be a
need for the RWSEM_OWNER_UNKNOWN magic and it can be removed.

Cheers,
Longman

RWSEM_OWNER_UNKNOWN