Messages in this thread | | | Date | Wed, 5 Apr 2017 01:15:01 +0900 | From | Sergey Senozhatsky <> | 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
| |