Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump from NMI context

[Konstantin Khlebnikov]

On 15.07.2019 10:54, Petr Mladek wrote:
> On Mon 2019-07-15 11:33:38, Sergey Senozhatsky wrote:
> > On (07/13/19 17:03), Konstantin Khlebnikov wrote:
> > > > We call kmsg_dump(KMSG_DUMP_PANIC) after smp_send_stop()
> > >
> > > Indeed, panic is especially handled and looks fine.
>
> Just to get a picture. What other situations are we talking about,
> please?
>
> oops_exit() is one candidate. The other callers seem to be working
> in normal context.

Oops in NMI mostly. Also I've screwed up several times with NMI watchdog
and dumping log at setting taint.

> > > Sanity check in my patch could be relaxed:
> > >
> > >         if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi()))
> > >                 return;
> >
> > How critical kmsg_dump() is? We deadlock only if NMI->kmsg_dump()
> > happens on the CPU which already holds the logbuf_lock; in any
> > other case logbuf_lock is owned by another CPU which is expected
> > to unlock it eventually. So it doesn't look like disabling all
> > NMI->kmsg_dump() is the only way to fix it.
> >
> > When we lock logbuf_lock we increment per-CPU printk_context
> > (PRINTK_SAFE_CONTEXT_MASK bits); when we unlock logbuf_lock
> > we decrement printk_context. Thus we always can tell if the
> > logbuf_lock was locked on the very same CPU - this_cpu printk_context
> > has PRINTK_SAFE_CONTEXT_MASK bits sets - and we are about to deadlock
> > in a nested context (NMI), or the lock was locked on another CPU and
> > it's "safe" to spin on logbuf_lock and wait for logbuf_lock to become
> > available.
>
> This sounds familiar. I think that we did not consider it safe in the
> end, see the commit 03fc7f9c99c1e7ae29 ("printk/nmi: Prevent deadlock
> when accessing the main log buffer in NMI").
>
> If the problem is only with Oops then the 2nd propose might be
> acceptable. The system will either try to continue or it will end
> up in panic() anyway.
>
> Well, WARN() looks like an overkill, especially if there is only
> one possible caller that prints the stack anyway. I would personally
> do not print any message and do just:
>
> 	/*
> 	 * Prevent deadlock on logbuf_lock. It is safe only
> 	 * in panic() after smp_send_stop() and resetting
> 	 * the lock.
> 	 */
> 	if (in_nmi() && reason != KMSG_DUMP_PANIC)
> 		return;

WARN_ON_ONCE is useful timesaver in debugging.
It's better to know when happens something that shouldn't.

> Best Regards,
> Petr