Re: [PATCH] Fix a dead loop in async_synchronize_full()

[Christian Kujau]

On Mon, 2 Jul 2012 at 14:04, Li Zhong wrote:
> This patch tries to fix a dead loop in  async_synchronize_full(), which
> could be seen when preemption is disabled on a single cpu machine. 

This helps in my case[0], will this patch be included in -rc6?

Thanks,
Christian.

[0] https://lists.ozlabs.org/pipermail/linuxppc-dev/2012-July/098922.html


> void async_synchronize_full(void)
> {
>         do {
>                 async_synchronize_cookie(next_cookie);
>         } while (!list_empty(&async_running) || !
> list_empty(&async_pending));
> }
> 
> async_synchronize_cookie() calls async_synchronize_cookie_domain() with
> &async_running as the default domain to synchronize. 
> 
> However, there might be some works in the async_pending list from other
> domains. On a single cpu system, without preemption, there is no chance
> for the other works to finish, so async_synchronize_full() enters a dead
> loop. 
> 
> It seems async_synchronize_full() wants to synchronize all entries in
> all running lists(domains), so maybe we could just check the entry_count
> to know whether all works are finished. 
> 
> Currently, async_synchronize_cookie_domain() expects a non-NULL running
> list ( if NULL, there would be NULL pointer dereference ), so maybe a
> NULL pointer could be used as an indication for the functions to
> synchronize all works in all domains. 
> 
> Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
> Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/async.c |   13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/async.c b/kernel/async.c
> index bd0c168..32d8dc9 100644
> --- a/kernel/async.c
> +++ b/kernel/async.c
> @@ -86,6 +86,13 @@ static async_cookie_t  __lowest_in_progress(struct
> list_head *running)
>  {
>  	struct async_entry *entry;
>  
> +	if (!running) { /* just check the entry count */
> +		if (atomic_read(&entry_count))
> +			return 0; /* smaller than any cookie */
> +		else
> +			return next_cookie;
> +	}
> +
>  	if (!list_empty(running)) {
>  		entry = list_first_entry(running,
>  			struct async_entry, list);
> @@ -236,9 +243,7 @@ EXPORT_SYMBOL_GPL(async_schedule_domain);
>   */
>  void async_synchronize_full(void)
>  {
> -	do {
> -		async_synchronize_cookie(next_cookie);
> -	} while (!list_empty(&async_running) || !list_empty(&async_pending));
> +	async_synchronize_cookie_domain(next_cookie, NULL);
>  }
>  EXPORT_SYMBOL_GPL(async_synchronize_full);
>  
> @@ -258,7 +263,7 @@ EXPORT_SYMBOL_GPL(async_synchronize_full_domain);
>  /**
>   * async_synchronize_cookie_domain - synchronize asynchronous function
> calls within a certain domain with cookie checkpointing
>   * @cookie: async_cookie_t to use as checkpoint
> - * @running: running list to synchronize on
> + * @running: running list to synchronize on, NULL indicates all lists
>   *
>   * This function waits until all asynchronous function calls for the
>   * synchronization domain specified by the running list @list submitted
> -- 
> 1.7.9.5
> 
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-- 
BOFH excuse #192:

runaway cat on system.
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