On (10/06/16 16:56), Petr Mladek wrote:
[..]
> > there are many WARN_ON_* on
> > vprintk_alt()->alt_printk_log_store()->vsnprintf() path but they all
> > seem to be calling printk():
> >
> > - WARN_ON_ONCE() from vsnprintf()
> > - WARN_ONCE() from vsnprintf()->format_decode()
> > - WARN_ON vsnprintf()->set_field_width()
> > - WARN_ON from vsnprintf()->set_precision()
> > - WARN_ON from vsnprintf()->pointer()->flags_string()
> >
> > a side note, some of these WARNs are... 'funny'. e.g., to deadlock a
> > system it's enough to just pass an unsupported flag in format string.
> > vsnprintf() will
> > WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt)
> >
> > but the problem is that we are already in printk():
> >
> > printk()
> > raw_spin_lock(&logbuf_lock)
> > text_len = vscnprintf(text, sizeof(textbuf), fmt, args)
> > WARN_ONCE(1, "Please remove unsupported ...)
> > printk()
> > raw_spin_lock(&logbuf_lock) <<
> > deadlock
>
> Just for record. This vscnprintf() is called when logbuf_cpu is set.
> Therefore this particular recursion is not possible at the moment.
ah, indeed. thanks.
> Anyway, I agree that alt_printk_enter() calls might get nested.
> The direct vprintk_emit() calls are one reason. I guess that
> we will need to use the same counter/enter functions also
> for WARN_*DEFERRED(). And it would cause nesting as well.
>
> Therefore we need to be careful about loosing the original
> value of irq flags.
>
> The question is whether we need to store the flags in
> a per-CPU variable. We might also store it on the stack
> of the enter()/exit() function caller. I mean something like
yes, let's keep it on the stack. this particular implementation
was just an experiment, and I hate it. what I currently have in
my tree:
#define alt_printk_enter(flags) \
do { \
local_irq_save(flags); \
__alt_printk_enter(); \
} while (0)
#define alt_printk_exit(flags) \
do { \
__alt_printk_exit(); \
local_irq_restore(flags); \
} while (0)
-ss
