kdb: properly synchronize vkdb_printf() calls with other CPUs
authorPetr Mladek <pmladek@suse.com>
Wed, 14 Dec 2016 23:05:55 +0000 (15:05 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Thu, 15 Dec 2016 00:04:08 +0000 (16:04 -0800)
commitd5d8d3d0d4adcc3aec6e2e0fb656165014a712b7
tree090aa03f64ff9535310f073d3574fb692f720fa9
parentd1bd8ead126668a2d6c42d97cc3664e95b3fa1dc
kdb: properly synchronize vkdb_printf() calls with other CPUs

kdb_printf_lock does not prevent other CPUs from entering the critical
section because it is ignored when KDB_STATE_PRINTF_LOCK is set.

The problematic situation might look like:

CPU0 CPU1

vkdb_printf()
  if (!KDB_STATE(PRINTF_LOCK))
    KDB_STATE_SET(PRINTF_LOCK);
    spin_lock_irqsave(&kdb_printf_lock, flags);

vkdb_printf()
  if (!KDB_STATE(PRINTF_LOCK))

BANG: The PRINTF_LOCK state is set and CPU1 is entering the critical
section without spinning on the lock.

The problem is that the code tries to implement locking using two state
variables that are not handled atomically.  Well, we need a custom
locking because we want to allow reentering the critical section on the
very same CPU.

Let's use solution from Petr Zijlstra that was proposed for a similar
scenario, see
https://lkml.kernel.org/r/20161018171513.734367391@infradead.org

This patch uses the same trick with cmpxchg().  The only difference is
that we want to handle only recursion from the same context and
therefore we disable interrupts.

In addition, KDB_STATE_PRINTF_LOCK is removed.  In fact, we are not able
to set it a non-racy way.

Link: http://lkml.kernel.org/r/1480412276-16690-3-git-send-email-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
kernel/debug/kdb/kdb_io.c
kernel/debug/kdb/kdb_private.h