Messages in this thread | | | Date | Sat, 16 Jun 2012 08:28:36 -0700 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog_seq from going wild |
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On Sat, Jun 16, 2012 at 09:28:32PM +0800, Yuanhan Liu wrote: > On Sat, Jun 16, 2012 at 09:20:52PM +0800, Fengguang Wu wrote: > > On Sat, Jun 16, 2012 at 08:59:22PM +0800, Yuanhan Liu wrote: > > > On Sat, Jun 16, 2012 at 02:42:38PM +0200, Kay Sievers wrote: > > > > On Sat, 2012-06-16 at 12:40 +0800, Yuanhan Liu wrote: > > > > > Although syslog_seq and log_next_seq stuff are protected by logbuf_lock > > > > > spin log, it's not enough. Say we have two processes A and B, and let > > > > > syslog_seq = N, while log_next_seq = N + 1, and the two processes both > > > > > come to syslog_print at almost the same time. And No matter which > > > > > process get the spin lock first, it will increase syslog_seq by one, > > > > > then release spin lock; thus later, another process increase syslog_seq > > > > > by one again. In this case, syslog_seq is bigger than syslog_next_seq. > > > > > And latter, it would make: > > > > > wait_event_interruptiable(log_wait, syslog != log_next_seq) > > > > > don't wait any more even there is no new write comes. Thus it introduce > > > > > a infinite loop reading. > > > > > > > > Oh, multiple readers on the same shared file descriptor are not useful, > > > > but sure, that needs fixing. Thanks for tracking that down! > > > > > > > > Looks like the same issue existed in the original code already, it's > > > > just that it was granular at a single character level, and not a line, > > > > and the seqnum which icreases one-by-one, so the issue was hard to > > > > trigger. > > > > > > Yes, I think so, too. > > > > > > > > > > > We better make the mutexes interruptible, right? > > > > > > Yes, you are right. > > > > It might be better to do them in two standalone patches? > > One is a bug fix, the other improves user responsiveness. > > > > Either way, you may add my superficial > > > > Reviewed-by: Fengguang Wu <fengguang.wu@intel.com> > > Too late; I just sent it out ;( > > Anyway, thanks for your Reviewed-by very much. I hope commiter can add > this for me.
I will do so, thanks.
greg k-h
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