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lockdep: Print a nicer description for irq lock inversions
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: 
Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter 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.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>