lkml.org 
[lkml]   [2017]   [Aug]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] osq_lock: fix osq_lock queue corruption
From
Date
On 07/31/2017 10:54 PM, 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.
> + */
> + smp_wmb();
> +
> WRITE_ONCE(prev->next, node);
>
> /*
>

Hi Peter,

I have updated the change log and comments in code.

--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project

\
 
 \ /
  Last update: 2017-08-07 07:06    [W:0.092 / U:0.360 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site