linux-ext4 - Re: [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner

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Date:   Tue, 14 Jan 2020 13:17:45 -0500
From:   Waiman Long <longman@...hat.com>
To:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous
 owner

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@....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