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Date: Wed, 5 Apr 2017 01:15:01 +0900 From: Sergey Senozhatsky <sergey.senozhatsky@...il.com> To: Petr Mladek <pmladek@...e.com> Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, "Rafael J . Wysocki" <rjw@...ysocki.net>, Eric Biederman <ebiederm@...ssion.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> Subject: Re: [RFC][PATCHv2 3/8] printk: offload printing from wake_up_klogd_work_func() Hi Petr, sorry for the delay. On (03/31/17 16:56), Petr Mladek wrote: [..] > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > > index ab6b3b2a68c6..1927b5cb5cbe 100644 > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -2741,8 +2741,16 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work) > > * If trylock fails, someone else is doing the printing. > > * PRINTK_PENDING_OUTPUT bit is cleared by console_unlock(). > > */ > > - if (console_trylock()) > > - console_unlock(); > > + if (printk_kthread_enabled()) { > > + wake_up_process(printk_kthread); > > Note that the relation between printk_kthread_enabled() > and wake_up_process() is racy. The conditions might change > between these two calls. It looks fine here, well almost. > > The critical point is in vprintk_emit(). It must use the emergency > mode (call the consoles directly) when it is called from a process > that started the emergency mode. hm, we don't guarantee this. printk(), both in threaded and in emergency modes, can fail to acquire console_sem. > We could be more relaxed here. IMHO, the only sensitive situation > is if printk_deferred() is used in the emergency context. > We might want to use the emergency mode here as well but > it is not guaranteed. hm, I don't think any path does printk_emergency_begin() printk_deferred() printk_emergency_end() and expects logbuf output to be flushed by the time it does printk_emergency_end(). it's most likely something like this printk_emergency_begin() printk() printk_emergency_end() the expectations here are more reasonable, but still, no guarantees are provided (even in non-kthreaded printk mode). > A solution might be to add one more bit, e.g. > PRINTK_PENDING_EMERGENCY_OUTPUT. We should force the emergency mode > here when it is set. It should be cleared together with the normal > PRINTK_PENDING_OUTPUT. > > Or do you think that this is a corner case that we could > ignore for now? hm, I guess we don't really count on irq_work in emergency situations. but I need more time to think. good questions, Petr. -ss
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