Messages in this thread | | | Date | Thu, 1 Dec 2016 11:10:42 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC][PATCHv4 4/6] printk: report lost messages in printk safe/nmi contexts |
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On (11/25/16 12:07), Petr Mladek wrote: [..] > > +static void report_message_lost(atomic_t *num_lost, char *fmt) > > +{ > > + int lost = atomic_xchg(num_lost, 0); > > + > > + if (lost) { > > + char msg[56]; > > I would really like to avoid a hard coded buffer size. Such things > are likely to bite us in the future.
why would scnprintf() overflow.
> I thought about reshuffling a lot of logic, adding more wrappers, > ... But the solution might be easy in the end, see below. > > > + > > + scnprintf(msg, sizeof(msg), fmt, lost); > > + > > + printk_safe_flush_line(msg, strlen(msg)); > > This made my brain spin a lot. I wondered if it did what we wanted > and it was safe. > > On one hand, it is supposed to work because use exactly this > function in __printk_safe_flush() where you call this from. > > One question is if it does what we want in different contexts. > Let's look at it: > > 1. It calls printk_deferred() in NMI context. There is a risk > of a deadlock. But it is called only from > printk_safe_flush_on_panic() which is the last resort. Therefore > it does exactly what we want. > > 2. It calls printk()->printk_func()->vprintk_emit() in normal context. > It is what we want in normal context. > > 3. It calls printk()->printk_func()->v printk_safe() in printk_safe > context. This does not look correct. IMHO, this might happen > only printk_safe_flush_on_panic() and we want to use > printk_deferred() here as well. [..] > The easiest solution would be to simply call printk_deferred() > here. Everything will be deferred after the async printk() patchset > anyway. > > I would even use printk_deferred() in printk_safe_flush_line() > for each context. It is not optimal but it works very well > and it makes the code much more straightforward.
yes, good point. we can call deferred printk for anything there; or replace that in_nmi() check with the `printk_safe_context != 0' one, and then route the message via printk or printk_deferred.
[..] > > * Flush data from the associated per-CPU buffer. The function > > * can be called either via IRQ work or independently. > > @@ -147,6 +183,9 @@ static void __printk_safe_flush(struct irq_work *work) > > > > i = 0; > > more: > > + report_nmi_message_lost(); > > + report_safe_message_lost(); > > Please, move this at the end of this function.
ok.
-ss
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