Locking order inversion due to interrupts is a subtle problem.

When an irq lockiinversion discovered by lockdep it currently
reports something like:

[ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]

... and then prints out the locks that are involved, as back traces.

Judging by lkml feedback developers were routinely confused by what
a HARDIRQ->safe to unsafe issue is all about, and sometimes even
blew it off as a bug in lockdep.

It is not obvious when lockdep prints this message about a lock that
is never taken in interrupt context.

After explaining the problems that lockdep is reporting, I
decided to add a description of the problem in visual form. Now
the following is shown:

 ---
other info that might help us debug this:

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockA);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***

 ---

The above is the case when the unsafe lock is taken while
holding a lock taken in irq context. But when a lock is taken
that also grabs a unsafe lock, the call chain is shown:

 ---
other info that might help us debug this:

Chain exists of:
  &rq->lock --> lockA --> lockC

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockC);
                               local_irq_disable();
                               lock(&rq->lock);
                               lock(lockA);
  <Interrupt>
    lock(&rq->lock);

 *** DEADLOCK ***
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.132728798@goodmis.orgSigned-off-by: NIngo Molnar <mingo@elte.hu>