4.17-stable review patch. If anyone has any objections, please let me know.
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From: Sergey Senozhatsky sergey.senozhatsky.work@gmail.com
[ 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 rostedt@goodmis.org Cc: Peter Zijlstra peterz@infradead.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/printk/printk_safe.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/printk/printk_safe.c +++ b/kernel/printk/printk_safe.c @@ -281,7 +281,7 @@ 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;