• Petr Mladek's avatar
    printk/nmi: Prevent deadlock when accessing the main log buffer in NMI · 03fc7f9c
    Petr Mladek authored
    The commit 719f6a70 ("printk: Use the main logbuf in NMI
    when logbuf_lock is available") brought back the possible deadlocks
    in printk() and NMI.
    The check of logbuf_lock is done only in printk_nmi_enter() to prevent
    mixed output. But another CPU might take the lock later, enter NMI, and:
          + Both NMIs might be serialized by yet another lock, for example,
    	the one in nmi_cpu_backtrace().
          + The other CPU might get stopped in NMI, see smp_send_stop()
    	in panic().
    The only safe solution is to use trylock when storing the message
    into the main log-buffer. It might cause reordering when some lines
    go to the main lock buffer directly and others are delayed via
    the per-CPU buffer. It means that it is not useful in general.
    This patch replaces the problematic NMI deferred context with NMI
    direct context. It can be used to mark a code that might produce
    many messages in NMI and the risk of losing them is more critical
    than problems with eventual reordering.
    The context is then used when dumping trace buffers on oops. It was
    the primary motivation for the original fix. Also the reordering is
    even smaller issue there because some traces have their own time stamps.
    Finally, nmi_cpu_backtrace() need not longer be serialized because
    it will always us the per-CPU buffers again.
    Fixes: 719f6a70 ("printk: Use the main logbuf in NMI when logbuf_lock is available")
    Cc: stable@vger.kernel.org
    Link: http://lkml.kernel.org/r/20180627142028.11259-1-pmladek@suse.com
    To: Steven Rostedt <rostedt@goodmis.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
    Cc: linux-kernel@vger.kernel.org
    Cc: stable@vger.kernel.org
    Acked-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
nmi_backtrace.c 2.99 KB