RE: [PATCH] panic: move bust_spinlocks(0) after console_flush_on_panic() to avoid deadlocks

[Hoeun Ryu]

I misunderstood the cause of a deadlock.
I sent v2 fixing the commit message about the reason of the deadlock.
Please ignore this and review v2.
Thank you.

> -----Original Message-----
> From: Steven Rostedt [mailto:rostedt@xxxxxxxxxxx]
> Sent: Tuesday, June 05, 2018 10:44 AM
> To: Hoeun Ryu <hoeun.ryu@xxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Kees Cook
> <keescook@xxxxxxxxxxxx>; Borislav Petkov <bp@xxxxxxx>; Andi Kleen
> <ak@xxxxxxxxxxxxxxx>; Josh Poimboeuf <jpoimboe@xxxxxxxxxx>; Hoeun Ryu
> <hoeun.ryu@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Tejun Heo
> <tj@xxxxxxxxxx>; Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Subject: Re: [PATCH] panic: move bust_spinlocks(0) after
> console_flush_on_panic() to avoid deadlocks
> 
> On Mon,  4 Jun 2018 14:45:57 +0900
> Hoeun Ryu <hoeun.ryu@xxxxxxxxxxx> wrote:
> 
> > From: Hoeun Ryu <hoeun.ryu@xxxxxxx>
> >
> >  Many console device drivers hold the uart_port->lock spinlock with irq
> enabled
> > (using spin_lock()) while the device drivers are writing characters to
> their devices,
> > but the device drivers just try to hold the spin lock (using
> spin_trylock()) if
> > "oops_in_progress" is equal or greater than 1 to avoid deadlocks.
> >
> >  There is a case ocurring a deadlock related to the lock and
> oops_in_progress. A CPU
> > could be stopped by smp_send_stop() while it was holding the port lock
> because irq was
> > enabled. Once a CPU stops, it doesn't respond interrupts anymore and the
> lock stays
> > locked forever.
> >
> >  console_flush_on_panic() is called during panic() and it eventually
> holds the uart
> > lock but the lock is held by another stopped CPU and it is a deadlock.
> By moving
> > bust_spinlocks(0) after console_flush_on_panic(), let the console device
> drivers
> > think the Oops is still in progress to call spin_trylock() instead of
> spin_lock() to
> > avoid the deadlock.
> >
> > Signed-off-by: Hoeun Ryu <hoeun.ryu@xxxxxxx>
> > ---
> >  kernel/panic.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index 42e4874..b4063b6 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -233,8 +233,6 @@ void panic(const char *fmt, ...)
> >  	if (_crash_kexec_post_notifiers)
> >  		__crash_kexec(NULL);
> >
> > -	bust_spinlocks(0);
> > -
> >  	/*
> >  	 * We may have ended up stopping the CPU holding the lock (in
> >  	 * smp_send_stop()) while still having some valuable data in the
> console
> > @@ -246,6 +244,8 @@ void panic(const char *fmt, ...)
> >  	debug_locks_off();
> >  	console_flush_on_panic();
> >
> > +	bust_spinlocks(0);
> 
> Added a few more to Cc. This looks like it could have subtle
> side-effects. I'd like those that have been touching the code around
> here to have a look.
> 
> -- Steve
> 
> 
> > +
> >  	if (!panic_blink)
> >  		panic_blink = no_blink;
> >