Q. lockdep_assert_held() and lockdep_off/on()

From: J. R. Okajima
Date: Fri Apr 08 2011 - 09:20:22 EST

Hello Peter Zijlstra and Ingo Molnar,

May I ask you a question about the commit
f607c66 2009-08-02 lockdep: Introduce lockdep_assert_held()

In short, should lockdep_assert_held() support ->lockdep_recursion?

Its current definition is
#define lockdep_assert_held(l) WARN_ON(debug_locks && !lockdep_is_held(l))

When someone somewhere calls lockdep_off() and executes some memory
allocation or something, then the functions to shrink dentry cache
happens to run. And cond_resched_lock() in __shrink_dcache_sb() may
produce a false warning.

while (...) {

The function __shrink_dcache_sb() acquires dcache_lru_lock correctly and
comfirms it by cond_resched_lock() which calls lockdep_assert_held().
When the caller already called lockdep_off(), lock_is_held() always
return 0 which leads to WARN_ON(true). Obviously the warning is false

Setting FALSE to debug_locks may be one solution, but this variable
doesn't seem to expect to return to TRUE. So it is better for
lockdep_assert_held() to test ->lockdep_recursion too I think.

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 413c754..8658138 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -341,7 +341,9 @@ extern void lockdep_trace_alloc(gfp_t mask);

#define lockdep_depth(tsk) (debug_locks ? (tsk)->lockdep_depth : 0)

-#define lockdep_assert_held(l) WARN_ON_ONCE(debug_locks && !lockdep_is_held(l))
+#define lockdep_assert_held(l) WARN_ON_ONCE(debug_locks \
+ && !current->lockdep_recursion \
+ && !lockdep_is_held(l))

#else /* !LOCKDEP */

J. R. Okajima
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/