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

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

On (07/15/19 10:38), Konstantin Khlebnikov wrote:
> > > 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.
> 
> I see no users who dumps dmesg from NMI context except panic.
> This shouldn't happen. But might happen is something goes very wrong.

I see. OK, if NMI->kmsg_dump() never happens (except for panic) then
we are fine, I guess.

	-ss