futex: Fault/error injection capabilities

Although futexes are well known for being a royal pita,
we really have very little debugging capabilities - except
for relying on tglx's eye half the time.

By simply making use of the existing fault-injection machinery,
we can improve this situation, allowing generating artificial
uaddress faults and deadlock scenarios. Of course, when this is
disabled in production systems, the overhead for failure checks
is practically zero -- so this is very cheap at the same time.
Future work would be nice to now enhance trinity to make use of
this.

There is a special tunable 'ignore-private', which can filter
out private futexes. Given the tsk->make_it_fail filter and
this option, pi futexes can be narrowed down pretty closely.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <darren@dvhart.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Link: http://lkml.kernel.org/r/1435645562-975-3-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/fault-injection/fault-injection.txt

@@ -15,6 +15,10 @@ o fail_page_alloc
 
   injects page allocation failures. (alloc_pages(), get_free_pages(), ...)
 
+o fail_futex
+
+  injects futex deadlock and uaddr fault errors.
+
 o fail_make_request
 
   injects disk IO errors on devices permitted by setting
@@ -113,6 +117,12 @@ configuration of fault-injection capabilities.
 	specifies the minimum page allocation order to be injected
 	failures.
 
+- /sys/kernel/debug/fail_futex/ignore-private:
+
+	Format: { 'Y' | 'N' }
+	default is 'N', setting it to 'Y' will disable failure injections
+	when dealing with private (address space) futexes.
+
 o Boot option
 
 In order to inject faults while debugfs is not available (early boot time),
@@ -121,6 +131,7 @@ use the boot option:
 	failslab=
 	fail_page_alloc=
 	fail_make_request=
+	fail_futex=
 	mmc_core.fail_request=<interval>,<probability>,<space>,<times>
 
 How to add new fault injection capability