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
