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Mon, 19 Oct 2020 21:20:09 -0500 Received: (from bmarzins@localhost) by octiron.msp.redhat.com (8.14.9/8.14.9/Submit) id 09K2K8ck015682; Mon, 19 Oct 2020 21:20:08 -0500 Date: Mon, 19 Oct 2020 21:20:08 -0500 From: Benjamin Marzinski To: mwilck@suse.com Message-ID: <20201020022008.GK3384@octiron.msp.redhat.com> References: <20201016104501.8700-1-mwilck@suse.com> <20201016104501.8700-30-mwilck@suse.com> MIME-Version: 1.0 In-Reply-To: <20201016104501.8700-30-mwilck@suse.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-loop: dm-devel@redhat.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() X-BeenThere: dm-devel@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: device-mapper development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dm-devel-bounces@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Fri, Oct 16, 2020 at 12:45:01PM +0200, mwilck@suse.com wrote: > From: Martin Wilck > > 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 > --- > 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 -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel