On Mon, Oct 26, 2020 at 06:24:57PM +0100, Martin Wilck wrote: > On Mon, 2020-10-26 at 17:22 +0100, Martin Wilck wrote: > > On Mon, 2020-10-19 at 21:20 -0500, Benjamin Marzinski wrote: > > > On Fri, Oct 16, 2020 at 12:45:01PM +0200, mwilck@xxxxxxxx wrote: > > > > From: Martin Wilck <mwilck@xxxxxxxx> > > > > > > > > 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(). > > > > If we do this, we might as well use the variable "la" itself for > > that, > > and make sure it's only accessed under the lock. It'd be fine, > > because > > la is used if and only if the log thread is active, and spare us > > another variable. I had actually considered that, thought it was too > > invasive for the already big series. If you prefer this way, I can do > > it. > > OTOH, we take logev_lock in log_safe() anyway (to set > log_messages_pending). I doubt that it makes a big difference if we > take the two locks sequentially, or nested. The previous code actually > took the logev_lock twice, before and after logq_lock. Assuming that > contention is rather rare, I believe my code might actually perform > better than before. > > In your previous review > https://www.redhat.com/archives/dm-devel/2020-September/msg00631.html > you pointed out that you considered it important that log_safe() works > even after the thread was stopped. Making this work implies that > log_safe() needs to check if the thread is up. So we either have to > take logev_lock twice, or take logq_lock while holding logev_lock. > > Bottom line: I think my patch is correct. We could add another patch on > top that moves logq_lock() into log.c, protecting the "la" variable, > but the nesting would still need to be the same. > > Does this make sense? No. Maybe I'm just being dumb, but couldn't log safe: - grab the logq_lock - check if the log_area is usable. We can use la for this. - If not, unlock, write to syslog and return - If so, you know that flush_logqueue() hasn't been run by log_thread_stop() yet, meaning anything you add to the log will get flushed by flush_logqueue, so enqueue the message -unlock logq_lock and lock logev_lock -signal that there are messages pending. If nobody is listening on the the other side, who cares, because log_thread_stop() will still flush the enqueued message -unlock logev_lock Am I missing something? -Ben > > Regards, > Martin > > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
