[stgt] [PATCH v2] tgtd: refresh ready fds of event loop after event deletion

[Hitoshi Mitake]

At Wed, 19 Feb 2014 13:21:59 +0900,
Hitoshi Mitake wrote:
> 
> Current main event loop (event_loop()) of tgtd has a possibility of
> segmentation fault. The problem is caused by the below sequence:
> 
> 1. Event A, B are ready so epoll_wait(2) returns.
> 2. The handler of the event A is called. In the event handler, the
>    event B is deleted with tgt_event_del()
> 3. event_loop() tries to call the handler of the event B. It causes
>    segfault because the event struct is already removed and freed.
> 
> For avoid this problem, this patch adds a new global variable
> event_need_refresh. If the value of this variable is 1, event_loop()
> calls epoll_wait(2) again for refreshing ready fd list. This patch
> also lets tgt_event_del() to turn on the flag in its tail.
> 
> For example, we can produce segfault of tgtd under heavy load. Below
> is a backtrace obtained from the core file:
> (gdb) bt

Sorry, the lines of backtrace seems to be removed by git because they
start with '#'. Below is the backtrace:
#0  0x0000000000000000 in ?? ()
#1  0x0000000000411419 in event_loop () at tgtd.c:414
#2  0x0000000000411b65 in main (argc=<value optimized out>, argv=<value optimized out>) at tgtd.c:591

Thanks,
Hitoshi

> 
> To be honest, I still don't find an event handler which calls
> tgt_event_del() for other fds. But with this modification, the above
> segfault is avoided. The change seems to be effective.
> 
> Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> ---
>  usr/tgtd.c |    9 +++++++++
>  usr/tgtd.h |    2 ++
>  2 files changed, 11 insertions(+)
> 
> v2: use the existing label "retry" instead of adding a new label
> 
> diff --git a/usr/tgtd.c b/usr/tgtd.c
> index 50e1c83..04b31dc 100644
> --- a/usr/tgtd.c
> +++ b/usr/tgtd.c
> @@ -212,6 +212,8 @@ static struct event_data *tgt_event_lookup(int fd)
>  	return NULL;
>  }
>  
> +int event_need_refresh;
> +
>  void tgt_event_del(int fd)
>  {
>  	struct event_data *tev;
> @@ -229,6 +231,8 @@ void tgt_event_del(int fd)
>  
>  	list_del(&tev->e_list);
>  	free(tev);
> +
> +	event_need_refresh = 1;
>  }
>  
>  int tgt_event_modify(int fd, int events)
> @@ -426,6 +430,11 @@ retry:
>  		for (i = 0; i < nevent; i++) {
>  			tev = (struct event_data *) events[i].data.ptr;
>  			tev->handler(tev->fd, events[i].events, tev->data);
> +
> +			if (event_need_refresh) {
> +				event_need_refresh = 0;
> +				goto retry;
> +			}
>  		}
>  	}
>  
> diff --git a/usr/tgtd.h b/usr/tgtd.h
> index 8a25521..46b02af 100644
> --- a/usr/tgtd.h
> +++ b/usr/tgtd.h
> @@ -405,4 +405,6 @@ struct service_action *
>  find_service_action(struct service_action *service_action,
>  		    uint32_t action);
>  
> +extern int event_need_refresh;
> +
>  #endif
> -- 
> 1.7.10.4
> 
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