Re: [dm-devel] [PATCH v2 01/29] multipathd: uxlsnr: avoid deadlock on exit

[Benjamin Marzinski <bmarzins@redhat.com>]

On Fri, Oct 16, 2020 at 12:44:33PM +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> The uxlsnr wouldn't always release the client lock when cancelled,
> causing a deadlock in uxsock_cleanup(). While this hasn't been
> caused by commit 3d611a2, the deadlock seems to have become much
> more likely after that patch. Solving this means that we have to
> treat reallocation failure of the pollfd array differently.
> We will now just ignore any clients above the last valid pfd index.
> That's a minor problem, as we're in an OOM situation anyway.
> 
> Moreover, client_lock is not a "struct lock", but a plain
> pthread_mutex_t.
> 
> Fixes: 3d611a2 ("multipathd: cancel threads early during shutdown")

Oops. Forgot to send this one.

Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  multipathd/uxlsnr.c | 24 ++++++++++++++----------
>  1 file changed, 14 insertions(+), 10 deletions(-)
> 
> diff --git a/multipathd/uxlsnr.c b/multipathd/uxlsnr.c
> index 1c5ce9d..ce2b680 100644
> --- a/multipathd/uxlsnr.c
> +++ b/multipathd/uxlsnr.c
> @@ -35,6 +35,7 @@
>  #include "config.h"
>  #include "mpath_cmd.h"
>  #include "time-util.h"
> +#include "util.h"
>  
>  #include "main.h"
>  #include "cli.h"
> @@ -116,7 +117,7 @@ static void _dead_client(struct client *c)
>  
>  static void dead_client(struct client *c)
>  {
> -	pthread_cleanup_push(cleanup_lock, &client_lock);
> +	pthread_cleanup_push(cleanup_mutex, &client_lock);
>  	pthread_mutex_lock(&client_lock);
>  	_dead_client(c);
>  	pthread_cleanup_pop(1);
> @@ -302,10 +303,11 @@ void * uxsock_listen(uxsock_trigger_fn uxsock_trigger, long ux_sock,
>  	sigdelset(&mask, SIGUSR1);
>  	while (1) {
>  		struct client *c, *tmp;
> -		int i, poll_count, num_clients;
> +		int i, n_pfds, poll_count, num_clients;
>  
>  		/* setup for a poll */
>  		pthread_mutex_lock(&client_lock);
> +		pthread_cleanup_push(cleanup_mutex, &client_lock);
>  		num_clients = 0;
>  		list_for_each_entry(c, &clients, node) {
>  			num_clients++;
> @@ -322,14 +324,13 @@ void * uxsock_listen(uxsock_trigger_fn uxsock_trigger, long ux_sock,
>  						sizeof(struct pollfd));
>  			}
>  			if (!new) {
> -				pthread_mutex_unlock(&client_lock);
>  				condlog(0, "%s: failed to realloc %d poll fds",
>  					"uxsock", 2 + num_clients);
> -				sched_yield();
> -				continue;
> +				num_clients = old_clients;
> +			} else {
> +				old_clients = num_clients;
> +				polls = new;
>  			}
> -			old_clients = num_clients;
> -			polls = new;
>  		}
>  		polls[0].fd = ux_sock;
>  		polls[0].events = POLLIN;
> @@ -347,11 +348,14 @@ void * uxsock_listen(uxsock_trigger_fn uxsock_trigger, long ux_sock,
>  			polls[i].fd = c->fd;
>  			polls[i].events = POLLIN;
>  			i++;
> +			if (i >= 2 + num_clients)
> +				break;
>  		}
> -		pthread_mutex_unlock(&client_lock);
> +		n_pfds = i;
> +		pthread_cleanup_pop(1);
>  
>  		/* most of our life is spent in this call */
> -		poll_count = ppoll(polls, i, &sleep_time, &mask);
> +		poll_count = ppoll(polls, n_pfds, &sleep_time, &mask);
>  
>  		handle_signals(false);
>  		if (poll_count == -1) {
> @@ -384,7 +388,7 @@ void * uxsock_listen(uxsock_trigger_fn uxsock_trigger, long ux_sock,
>  		}
>  
>  		/* see if a client wants to speak to us */
> -		for (i = 2; i < num_clients + 2; i++) {
> +		for (i = 2; i < n_pfds; i++) {
>  			if (polls[i].revents & POLLIN) {
>  				struct timespec start_time;
>  
> -- 
> 2.28.0

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