Re: [PATCH 11/13] Add a discussion on why spin_is_locked() is badto spinlocks.txt

[Wolfram Sang]

On Tue, Mar 27, 2012 at 05:47:15PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  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))

'&my_lock' instead of '!my_lock' probably.

> +
> +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.

I'd suggest to move this sentence above the code example. On first read,
I was confused what the code should tell me regarding annotations :)

> +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.

At this point, I was wondering when it actually can be used? Otherwise
it probably would have been removed from the kernel or marked
deprecated, I'd think?

Regards,

   Wolfram

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