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SubjectRE: [PATCH v2] panic: move bust_spinlocks(0) after console_flush_on_panic() to avoid deadlocks
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sergey.senozhatsky.work@gmail.com
pmladek@suse.com

Please review this patch.

> -----Original Message-----
> From: Hoeun Ryu [mailto:hoeun.ryu@lge.com.com]
> Sent: Tuesday, June 05, 2018 11:19 AM
> To: Andrew Morton <akpm@linux-foundation.org>; Kees Cook
> <keescook@chromium.org>; Andi Kleen <ak@linux.intel.com>; Borislav Petkov
> <bp@suse.de>; Thomas Gleixner <tglx@linutronix.de>; Steven Rostedt
(VMware)
> <rostedt@goodmis.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>; Tejun Heo <tj@kernel.org>;
> Vitaly Kuznetsov <vkuznets@redhat.com>; Hoeun Ryu <hoeun.ryu@lge.com>;
> linux-kernel@vger.kernel.org
> Subject: [PATCH v2] panic: move bust_spinlocks(0) after
> console_flush_on_panic() to avoid deadlocks
>
> From: Hoeun Ryu <hoeun.ryu@lge.com>
>
> Many console device drivers hold the uart_port->lock spinlock with irq
> disabled
> (using spin_lock_irqsave()) while the device drivers are writing
> characters to their
> devices, but the device drivers just try to hold the spin lock (using
> spin_trylock_irqsave()) instead 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. If the
> kernel lockup detector calls panic() while the device driver is holding
> the lock,
> it can cause a deadlock because panic() eventually calls console_unlock()
> and tries
> to hold the lock. Here is an example.
>
> CPU0
>
> local_irq_save()
> .
> foo()
> bar()
> . // foo() + bar() takes long
time
> printk()
> console_unlock()
> call_console_drivers() // close to watchdog
threshold
> some_slow_console_device_write() // device driver
code
> spin_lock_irqsave(uart->lock) // acquire uart spin
lock
> slow-write()
> watchdog_overflow_callback() // watchdog expired and call
> panic()
> panic()
> bust_spinlocks(0) // now, oops_in_progress = 0
> console_flush_on_panic()
> console_unlock()
> call_console_drivers()
> some_slow_console_device_write()
> spin_lock_irqsave(uart->lock)
> ^^^^ deadlock // we can use
> spin_trylock_irqsave()
>
> console_flush_on_panic() is called in panic() and it eventually holds the
> uart
> lock but the lock is held by the preempted CPU (the same CPU in NMI
> context) 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_irqsave()
> instead of
> spin_lock_irqsave() to avoid the deadlock.
>
> CPU0
>
> watchdog_overflow_callback() // watchdog expired and
> call panic()
> panic()
> console_flush_on_panic()
> console_unlock()
> call_console_drivers()
> some_slow_console_device_write()
> spin_trylock_irqsave(uart->lock) // oops_in_progress = 1
> ^^^^ use trylock, no deadlock
> bust_spinlocks(0) // now,
oops_in_progress =
> 0
>
> Signed-off-by: Hoeun Ryu <hoeun.ryu@lge.com>
> ---
> v2: fix commit message on the reason of a deadlock, no code change.
>
> 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);
> +
> if (!panic_blink)
> panic_blink = no_blink;
>
> --
> 2.1.4

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