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 <prs...@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.and...@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.
> 

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