kdb: call vkdb_printf() from vprintk_default() only when wanted

kdb_trap_printk allows to pass normal printk() messages to kdb via
vkdb_printk().  For example, it is used to get backtrace using the
classic show_stack(), see kdb_show_stack().

vkdb_printf() tries to avoid a potential infinite loop by disabling the
trap.  But this approach is racy, for example:

CPU1					CPU2

vkdb_printf()
  // assume that kdb_trap_printk == 0
  saved_trap_printk = kdb_trap_printk;
  kdb_trap_printk = 0;

					kdb_show_stack()
					  kdb_trap_printk++;

Problem1: Now, a nested printk() on CPU0 calls vkdb_printf()
	  even when it should have been disabled. It will not
	  cause a deadlock but...

   // using the outdated saved value: 0
   kdb_trap_printk = saved_trap_printk;

					  kdb_trap_printk--;

Problem2: Now, kdb_trap_printk == -1 and will stay like this.
   It means that all messages will get passed to kdb from
   now on.

This patch removes the racy saved_trap_printk handling.  Instead, the
recursion is prevented by a check for the locked CPU.

The solution is still kind of racy.  A non-related printk(), from
another process, might get trapped by vkdb_printf().  And the wanted
printk() might not get trapped because kdb_printf_cpu is assigned.  But
this problem existed even with the original code.

A proper solution would be to get_cpu() before setting kdb_trap_printk
and trap messages only from this CPU.  I am not sure if it is worth the
effort, though.

In fact, the race is very theoretical.  When kdb is running any of the
commands that use kdb_trap_printk there is a single active CPU and the
other CPUs should be in a holding pen inside kgdb_cpu_enter().

The only time this is violated is when there is a timeout waiting for
the other CPUs to report to the holding pen.

Finally, note that the situation is a bit schizophrenic.  vkdb_printf()
explicitly allows recursion but only from KDB code that calls
kdb_printf() directly.  On the other hand, the generic printk()
recursion is not allowed because it might cause an infinite loop.  This
is why we could not hide the decision inside vkdb_printf() easily.

Link: http://lkml.kernel.org/r/1480412276-16690-4-git-send-email-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: 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>

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