[dm-devel] [PATCH v3 18/29] libmultipath: fix log_thread startup and teardown

[Benjamin Marzinski]

On Wed, Dec 16, 2020 at 07:16:57PM +0100, mwilck at suse.com wrote:
> From: Martin Wilck <mwilck at suse.com>
> 
> This fixes several issues with the log_thread. First, the running flag
> logq_running should be set by the thread itself, not by
> log_thread_start()/_stop(). Second, the thread was both cancelled and
> terminated via a flag (again, logq_running). It's sufficient to just cancel
> and join it. Third, the locking wasn't cancel-safe in some places. Forth,
> log_thread_start() didn't wait for startup properly. Fifth, using (pthread_t)0
> is wrong (pthread_t is opaque; there's no guarantee that 0 is not a valid
> pthread_t value). Sixth, pthread_cancel() was called under logq_lock, which
> doesn't make sense to me.
> 
Reviewed-by: Benjamin Marzinski <bmarzins at redhat.com>
> Signed-off-by: Martin Wilck <mwilck at suse.com>
> ---
>  libmultipath/log_pthread.c | 62 +++++++++++++++++++++++++++-----------
>  1 file changed, 45 insertions(+), 17 deletions(-)
> 
> diff --git a/libmultipath/log_pthread.c b/libmultipath/log_pthread.c
> index 0c327ff..3a2566a 100644
> --- a/libmultipath/log_pthread.c
> +++ b/libmultipath/log_pthread.c
> @@ -13,6 +13,7 @@
>  #include "log_pthread.h"
>  #include "log.h"
>  #include "lock.h"
> +#include "util.h"
>  
>  static pthread_t log_thr;
>  
> @@ -56,35 +57,52 @@ static void flush_logqueue (void)
>  	} while (empty == 0);
>  }
>  
> +static void cleanup_log_thread(__attribute((unused)) void *arg)
> +{
> +	logdbg(stderr, "log thread exiting");
> +	pthread_mutex_lock(&logev_lock);
> +	logq_running = 0;
> +	pthread_mutex_unlock(&logev_lock);
> +}
> +
>  static void * log_thread (__attribute__((unused)) void * et)
>  {
>  	int running;
>  
>  	pthread_mutex_lock(&logev_lock);
> -	logq_running = 1;
> +	running = logq_running;
> +	if (!running)
> +		logq_running = 1;
> +	pthread_cond_signal(&logev_cond);
>  	pthread_mutex_unlock(&logev_lock);
> +	if (running)
> +		/* already started */
> +		return NULL;
> +	pthread_cleanup_push(cleanup_log_thread, NULL);
>  
>  	mlockall(MCL_CURRENT | MCL_FUTURE);
>  	logdbg(stderr,"enter log_thread\n");
>  
>  	while (1) {
>  		pthread_mutex_lock(&logev_lock);
> -		if (logq_running && !log_messages_pending)
> +		pthread_cleanup_push(cleanup_mutex, &logev_lock);
> +		while (!log_messages_pending)
> +			/* this is a cancellation point */
>  			pthread_cond_wait(&logev_cond, &logev_lock);
>  		log_messages_pending = 0;
> -		running = logq_running;
> -		pthread_mutex_unlock(&logev_lock);
> -		if (!running)
> -			break;
> +		pthread_cleanup_pop(1);
> +
>  		flush_logqueue();
>  	}
> +	pthread_cleanup_pop(1);
>  	return NULL;
>  }
>  
>  void log_thread_start (pthread_attr_t *attr)
>  {
> -	logdbg(stderr,"enter log_thread_start\n");
> +	int running = 0;
>  
> +	logdbg(stderr,"enter log_thread_start\n");
>  	pthread_mutex_init(&logq_lock, NULL);
>  	pthread_mutex_init(&logev_lock, NULL);
>  	pthread_cond_init(&logev_cond, NULL);
> @@ -93,7 +111,15 @@ void log_thread_start (pthread_attr_t *attr)
>  		fprintf(stderr,"can't initialize log buffer\n");
>  		exit(1);
>  	}
> -	if (pthread_create(&log_thr, attr, log_thread, NULL)) {
> +
> +	pthread_mutex_lock(&logev_lock);
> +	pthread_cleanup_push(cleanup_mutex, &logev_lock);
> +	if (!pthread_create(&log_thr, attr, log_thread, NULL))
> +		while (!(running = logq_running))
> +			pthread_cond_wait(&logev_cond, &logev_lock);
> +	pthread_cleanup_pop(1);
> +
> +	if (!running) {
>  		fprintf(stderr,"can't start log thread\n");
>  		exit(1);
>  	}
> @@ -112,23 +138,25 @@ void log_thread_reset (void)
>  
>  void log_thread_stop (void)
>  {
> +	int running;
> +
>  	if (!la)
>  		return;
>  
>  	logdbg(stderr,"enter log_thread_stop\n");
>  
>  	pthread_mutex_lock(&logev_lock);
> -	logq_running = 0;
> -	pthread_cond_signal(&logev_cond);
> -	pthread_mutex_unlock(&logev_lock);
> -
> -	pthread_mutex_lock(&logq_lock);
> -	pthread_cancel(log_thr);
> -	pthread_mutex_unlock(&logq_lock);
> -	pthread_join(log_thr, NULL);
> -	log_thr = (pthread_t)0;
> +	pthread_cleanup_push(cleanup_mutex, &logev_lock);
> +	running = logq_running;
> +	if (running) {
> +		pthread_cancel(log_thr);
> +		pthread_cond_signal(&logev_cond);
> +	}
> +	pthread_cleanup_pop(1);
>  
>  	flush_logqueue();
> +	if (running)
> +		pthread_join(log_thr, NULL);
>  
>  	pthread_mutex_destroy(&logq_lock);
>  	pthread_mutex_destroy(&logev_lock);
> -- 
> 2.29.0