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

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (07/13/19 17:03), Konstantin Khlebnikov wrote:
> > We call kmsg_dump(KMSG_DUMP_PANIC) after smp_send_stop() and after
> > printk_safe_flush_on_panic(). printk_safe_flush_on_panic() resets
> > the state of logbuf_lock, so logbuf_lock, in general case, should
> > be unlocked by the time we call kmsg_dump(KMSG_DUMP_PANIC).
> > Even for nested contexts.
> >
> >         CPU0
> >         printk()
> >          logbuf_lock_irqsave(flags)
> >           -> NMI
> >            panic()
> >             smp_send_stop()
> >              printk_safe_flush_on_panic()
> >               raw_spin_lock_init(&logbuf_lock) << reinit >>
> >             kmsg_dump(KMSG_DUMP_PANIC)
> >              logbuf_lock_irqsave(flags)        << expected to be OK >>
> >
> > So do we have strong reasons to disable NMI->panic->kmsg_dump(DUMP_PANIC)?
> >
> > Other kmsg_dump(), maybe, can experience some troubles sometimes,
> > need to check that.
> 
> Indeed, panic is especially handled and looks fine.
> 
> 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.

	-ss