From 9e95175e16a298f8c5da142a80da442854cfdb96 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Thu, 3 May 2012 15:44:10 +1000 Subject: [PATCH] spinlocks.txt: add a discussion on why spin_is_locked() is bad Signed-off-by: Andi Kleen Cc: Wolfram Sang Signed-off-by: Andrew Morton --- Documentation/spinlocks.txt | 43 +++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt index 9dbe885ecd8d..9a020e1b0b3b 100644 --- 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 -- 2.39.5