From 8dcf2dbf65efd17512db17526909de05daa0cff2 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Wed, 30 May 2018 16:03:50 +0900 Subject: [PATCH] printk: drop in_nmi check from printk_safe_flush_on_panic() [ Upstream commit 554755be08fba31c74f66b82a485e5513205af84 ] Drop the in_nmi() check from printk_safe_flush_on_panic() and attempt to re-init (IOW unlock) locked logbuf spinlock from panic CPU regardless of its context. Otherwise, theoretically, we can deadlock on logbuf trying to flush per-CPU buffers: a) Panic CPU is running in non-NMI context b) Panic CPU sends out shutdown IPI via reboot vector c) Panic CPU fails to stop all remote CPUs d) Panic CPU sends out shutdown IPI via NMI vector One of the CPUs that we bring down via NMI vector can hold logbuf spin lock (theoretically). Link: http://lkml.kernel.org/r/20180530070350.10131-1-sergey.senozhatsky@gmail.com To: Steven Rostedt Cc: Peter Zijlstra Cc: linux-kernel@vger.kernel.org Signed-off-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- kernel/printk/printk_safe.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c index d989cc2..64825b2 100644 --- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -284,7 +284,7 @@ void printk_safe_flush_on_panic(void) * Make sure that we could access the main ring buffer. * Do not risk a double release when more CPUs are up. */ - if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) { + if (raw_spin_is_locked(&logbuf_lock)) { if (num_online_cpus() > 1) return; -- 2.7.4