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Message-Id: <1332895637-32572-12-git-send-email-andi@firstfloor.org> Date: Tue, 27 Mar 2012 17:47:15 -0700 From: Andi Kleen <andi@...stfloor.org> To: akpm@...ux-foundation.org Cc: linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com> Subject: [PATCH 11/13] Add a discussion on why spin_is_locked() is bad to spinlocks.txt From: Andi Kleen <ak@...ux.intel.com> Signed-off-by: Andi Kleen <ak@...ux.intel.com> --- Documentation/spinlocks.txt | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 43 insertions(+), 0 deletions(-) diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt index 9dbe885..1787229 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 -- 1.7.7.6 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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