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From: Benjamin Marzinski <bmarzins@redhat.com>
To: mwilck@suse.com
Cc: lixiaokeng@huawei.com, dm-devel@redhat.com
Subject: Re: [dm-devel] [PATCH v2 29/29] libmultipath: fix race between log_safe and log_thread_stop()
Date: Mon, 19 Oct 2020 21:20:08 -0500
Message-ID: <20201020022008.GK3384@octiron.msp.redhat.com> (raw)
In-Reply-To: <20201016104501.8700-30-mwilck@suse.com>

On Fri, Oct 16, 2020 at 12:45:01PM +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> log_safe() could race with log_thread_stop(); simply
> checking the value of log_thr has never been safe. By converting the
> mutexes to static initializers, we avoid having to destroy them, and thus
> possibly accessing a destroyed mutex in log_safe(). Furthermore, taking
> both the logev_lock and the logq_lock makes sure the logarea isn't freed
> while we are writing to it.
> 

I don't see any problems with this, but I also don't think it's necssary
to hold the log thread lock (logev_lock), just to add a message to the
queue. It seems like protecting the log queue is the job of logq_lock.
As long as log_safe() enqueues the message before flush_logqueue() is
called in log_thread_stop(), it should be fine. This could be solved by
simply having a static variable in log_pthread.c named something like
log_area_enabled, that always accessed while holding the logq_lock, and
is set to true when the log_area is created, and set to false just
before calling the flush_logqueue() in log_thread_stop(). Then log_safe
just needs to check this before calling log_enqueue(). If it's true,
then logged messages will get flushed, so it can safely enqueue the
message. If it's false, then log_safe should call vsyslog() directly. 

In fairness, if the argument is that we should try to only lock what's
absolutely necessary, then multipath has far worse offenders than
logev_lock. So I don't feel super strongly about this, if there's a
reason why you like your way better.

-Ben

> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  libmultipath/log_pthread.c | 39 ++++++++++++++++++++++++--------------
>  1 file changed, 25 insertions(+), 14 deletions(-)
> 
> diff --git a/libmultipath/log_pthread.c b/libmultipath/log_pthread.c
> index 3c73941..91c9c19 100644
> --- a/libmultipath/log_pthread.c
> +++ b/libmultipath/log_pthread.c
> @@ -17,31 +17,42 @@
>  
>  static pthread_t log_thr;
>  
> -static pthread_mutex_t logq_lock;
> -static pthread_mutex_t logev_lock;
> -static pthread_cond_t logev_cond;
> +/* logev_lock must not be taken with logq_lock held */
> +static pthread_mutex_t logq_lock = PTHREAD_MUTEX_INITIALIZER;
> +static pthread_mutex_t logev_lock = PTHREAD_MUTEX_INITIALIZER;
> +static pthread_cond_t logev_cond = PTHREAD_COND_INITIALIZER;
>  
>  static int logq_running;
>  static int log_messages_pending;
>  
>  void log_safe (int prio, const char * fmt, va_list ap)
>  {
> +	bool running;
> +
>  	if (prio > LOG_DEBUG)
>  		prio = LOG_DEBUG;
>  
> -	if (log_thr == (pthread_t)0) {
> -		vsyslog(prio, fmt, ap);
> -		return;
> -	}
> +	/*
> +	 * logev_lock protects logq_running. By holding it, we avoid a race
> +	 * with log_thread_stop() -> log_close(), which would free the logarea.
> +	 */
> +	pthread_mutex_lock(&logev_lock);
> +	pthread_cleanup_push(cleanup_mutex, &logev_lock);
> +	running = logq_running;
>  
> -	pthread_mutex_lock(&logq_lock);
> -	log_enqueue(prio, fmt, ap);
> -	pthread_mutex_unlock(&logq_lock);
> +	if (running) {
> +		pthread_mutex_lock(&logq_lock);
> +		pthread_cleanup_push(cleanup_mutex, &logq_lock);
> +		log_enqueue(prio, fmt, ap);
> +		pthread_cleanup_pop(1);
>  
> -	pthread_mutex_lock(&logev_lock);
> -	log_messages_pending = 1;
> -	pthread_cond_signal(&logev_cond);
> -	pthread_mutex_unlock(&logev_lock);
> +		log_messages_pending = 1;
> +		pthread_cond_signal(&logev_cond);
> +	}
> +	pthread_cleanup_pop(1);
> +
> +	if (!running)
> +		vsyslog(prio, fmt, ap);
>  }
>  
>  static void flush_logqueue (void)
> -- 
> 2.28.0

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Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-16 10:44 [dm-devel] [PATCH v2 00/29] libmultipath: improve cleanup on exit mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 01/29] multipathd: uxlsnr: avoid deadlock " mwilck
2020-10-20 19:04   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 02/29] multipathd: Fix liburcu memory leak mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 03/29] multipathd: move handling of io_err_stat_attr into libmultipath mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 04/29] multipathd: move vecs desctruction into cleanup function mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 05/29] multipathd: make some globals static mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 06/29] multipathd: move threads destruction into separate function mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 07/29] multipathd: move conf " mwilck
2020-10-19 18:56   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 08/29] multipathd: move pid " mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 09/29] multipathd: close pidfile on exit mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 10/29] multipathd: add helper for systemd notification at exit mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 11/29] multipathd: child(): call cleanups in failure case, too mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 12/29] multipathd: unwatch_all_dmevents: check if waiter is initialized mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 13/29] multipathd: print error message if config can't be loaded mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 14/29] libmultipath: add libmp_dm_exit() mwilck
2020-10-19 19:07   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 15/29] multipathd: fixup libdm deinitialization mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 16/29] libmultipath: log_thread_stop(): check if logarea is initialized mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 17/29] multipathd: add cleanup_child() exit handler mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 18/29] libmultipath: fix log_thread startup and teardown mwilck
2020-10-19 20:00   ` Benjamin Marzinski
2020-10-26 13:58     ` Martin Wilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 19/29] multipathd: move cleanup_{prio, checkers, foreign} to libmultipath_exit mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 20/29] multipath: use atexit() for cleanup handlers mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 21/29] mpathpersist: " mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 22/29] multipath: fix leaks in check_path_valid() mwilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 23/29] multipath-tools: mpath-tools.supp: file with valgrind suppressions mwilck
2020-10-19 20:01   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 24/29] libmultipath: use libmp_verbosity to track verbosity mwilck
2020-10-19 20:38   ` Benjamin Marzinski
2020-10-26 14:47     ` Martin Wilck
2020-10-16 10:44 ` [dm-devel] [PATCH v2 25/29] libmultipath: introduce symbolic values for logsink mwilck
2020-10-16 20:13   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 26/29] libmultipath: simplify dlog() mwilck
2020-10-19 21:07   ` Benjamin Marzinski
2020-10-16 10:44 ` [dm-devel] [PATCH v2 27/29] multipathd: common code for "-k" and command args mwilck
2020-10-19 21:51   ` Benjamin Marzinski
2020-10-16 10:45 ` [dm-devel] [PATCH v2 28/29] multipathd: sanitize uxsock_listen() mwilck
2020-10-19 23:33   ` Benjamin Marzinski
2020-10-26 13:54     ` Martin Wilck
2020-10-16 10:45 ` [dm-devel] [PATCH v2 29/29] libmultipath: fix race between log_safe and log_thread_stop() mwilck
2020-10-20  2:20   ` Benjamin Marzinski [this message]
2020-10-26 16:22     ` Martin Wilck
2020-10-26 17:24       ` Martin Wilck
2020-11-03  0:11         ` Benjamin Marzinski
2020-11-04 12:36           ` Martin Wilck
2020-11-04 15:46             ` Benjamin Marzinski

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