spinlocks.txt: add a discussion on why spin_is_locked() is bad

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt
index 9dbe885ecd8d130e8a1e0300f05686dea59440c6..9a020e1b0b3b872c1bc01bef678d783ed681fea9 100644 (file)
--- a/Documentation/spinlocks.txt
+++ b/Documentation/spinlocks.txt
@@ -146,6 +146,49 @@ indeed), while write-locks need to protect themselves against interrupts.
 
 ----
 
+spin_is_locked is a bad idea
+
+spin_is_locked checks if a lock is currently hold.  On uniprocessor kernels
+it always returns 0. In general this function should be avoided because most
+uses of it are either redundant or broken.
+
+People often use spin_is_locked() to check if a particular lock is hold when a function
+is called to enforce a locking discipline, like
+
+       WARN_ON(!spin_is_locked(!my_lock))
+
+or
+
+       BUG_ON(!spin_is_locked(!my_lock))
+
+or some variant of those.
+
+This does not work on uniprocessor kernels because they will always fail.
+While there are ways around that they are ugly and not recommended.
+Better use lockdep_assert_held(). This also only checks on a lock debugging
+kernel (which you should occasionally run on your code anyways because
+it catches many more problems).
+
+In generally this would be better done with static annotation anyways
+(there's some support for it in sparse)
+
+       BUG_ON(spin_is_locked(obj->lock));
+       kfree(obj);
+
+Another usage is checking whether a lock is not hold when freeing an object.
+However this is redundant because lock debugging supports this anyways
+without explicit code. Just delete the BUG_ON.
+
+A third usage is to check in a console function if a lock is hold, to get
+a panic crash dump out even when some other thread died in it.
+This is better implemented with spin_try_lock() et.al. and a timeout.
+
+Other usages are usually simply races.
+
+In summary just don't use it.
+
+----
+
 Reference information:
 
 For dynamic initialization, use spin_lock_init() or rwlock_init() as