Date: Thu, 10 Aug 2017 06:40:05 +0200
From: Andrea Parri <parri.andrea@gmail.com>
To: Prateek Sood <prsood@codeaurora.org>, peterz@infradead.org,
        mingo@redhat.com
Cc: sramana@codeaurora.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] osq_lock: fix osq_lock queue corruption
Message-ID: <20170810044005.GA2907@andrea>
References: <1501521890-11661-1-git-send-email-prsood@codeaurora.org>
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On Mon, Jul 31, 2017 at 10:54:50PM +0530, Prateek Sood wrote:
> Fix ordering of link creation between node->prev and prev->next in
> osq_lock(). A case in which the status of optimistic spin queue is
> CPU6->CPU2 in which CPU6 has acquired the lock.
> 
>         tail
>           v
>   ,-. <- ,-.
>   |6|    |2|
>   `-' -> `-'
> 
> At this point if CPU0 comes in to acquire osq_lock, it will update the
> tail count.
> 
>   CPU2			CPU0
>   ----------------------------------
> 
> 				       tail
> 				         v
> 			  ,-. <- ,-.    ,-.
> 			  |6|    |2|    |0|
> 			  `-' -> `-'    `-'
> 
> After tail count update if CPU2 starts to unqueue itself from
> optimistic spin queue, it will find updated tail count with CPU0 and
> update CPU2 node->next to NULL in osq_wait_next().
> 
>   unqueue-A
> 
> 	       tail
> 	         v
>   ,-. <- ,-.    ,-.
>   |6|    |2|    |0|
>   `-'    `-'    `-'
> 
>   unqueue-B
> 
>   ->tail != curr && !node->next
> 
> If reordering of following stores happen then
> prev->next where prev being CPU2 would be updated to point to CPU0 node:
> 
> 				       tail
> 				         v
> 			  ,-. <- ,-.    ,-.
> 			  |6|    |2|    |0|
> 			  `-' -> `-' -> `-'
> 
>   osq_wait_next()
>     node->next <- 0
>     xchg(node->next, NULL)
> 
> 	       tail
> 	         v
>   ,-. <- ,-.    ,-.
>   |6|    |2|    |0|
>   `-'    `-'    `-'
> 
>   unqueue-C
> 
> At this point if next instruction
> WRITE_ONCE(next->prev, prev);
> in CPU2 path is committed before the update of CPU0 node->prev = prev then
> CPU0 node->prev will point to CPU6 node.
> 
> 	       tail
>     V----------. v
>   ,-. <- ,-.    ,-.
>   |6|    |2|    |0|
>   `-'    `-'    `-'
>      `----------^
> 
> At this point if CPU0 path's node->prev = prev is committed resulting
> in change of CPU0 prev back to CPU2 node. CPU2 node->next is NULL
> currently,
> 
> 				       tail
> 			                 v
> 			  ,-. <- ,-. <- ,-.
> 			  |6|    |2|    |0|
> 			  `-'    `-'    `-'
> 			     `----------^
> 
> so if CPU0 gets into unqueue path of osq_lock it will keep spinning
> in infinite loop as condition prev->next == node will never be true.
> 
> Signed-off-by: Prateek Sood <prsood@codeaurora.org>
> ---
>  kernel/locking/osq_lock.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index a316794..9f4afa3 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -109,6 +109,19 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  
>  	prev = decode_cpu(old);
>  	node->prev = prev;
> +
> +	/*
> +	 * osq_lock()			unqueue
> +	 *
> +	 * node->prev = prev            osq_wait_next()
> +	 * WMB                          MB
> +	 * prev->next = node            next->prev = prev //unqueue-C
> +	 *
> +	 * Here 'node->prev' and 'next->prev' are the same variable and we need
> +	 * to ensure these stores happen in-order to avoid corrupting the list.
> +	 */

The interested pattern/behavior remains somehow implicit in this snippet
(for example, as you described above, a load "reading from" the store to
prev->next is implicit in that osq_wait_next()); however I was unable to
come up with an alternative solution without complicating the comment.

Reviewed-by: Andrea Parri <parri.andrea@gmail.com>

  Andrea


> +	smp_wmb();
> +
>  	WRITE_ONCE(prev->next, node);
>  
>  	/*
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., 
> is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
>