printk: drop in_nmi check from printk_safe_flush_on_panic()
authorSergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Wed, 30 May 2018 07:03:50 +0000 (16:03 +0900)
committerPetr Mladek <pmladek@suse.com>
Tue, 5 Jun 2018 11:38:15 +0000 (13:38 +0200)
commit554755be08fba31c74f66b82a485e5513205af84
treeee752cf843b10a809f16ecd366895795c9faac55
parent988a35f8da1dec5a8cd2788054d1e717be61bf25
printk: drop in_nmi check from printk_safe_flush_on_panic()

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>
kernel/printk/printk_safe.c