Re: sparse warnings related to kref_put_lock() and refcount_dec_and_lock()

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Mon, Jun 27, 2022 at 11:15:17AM -0400, Alexander Aring wrote:
> Hi,
> 
> I recently converted to use kref_put_lock() in fs/dlm subsystem and
> now I get the following warning in sparse:
> 
> warning: context imbalance in 'put_rsb' - unexpected unlock
> 
> It seems sparse is not able to detect that there is a conditional
> requirement that the lock passed to kref_put_lock() (or also
> refcount_dec_and_lock()) is locked or not. I evaluate the return value
> to check if kref_put_lock() helds the lock and unlock it then. The
> idea is that the lock needs only to be held when the refcount is going
> to be zero.
> 
> It seems other users unlock the lock at the release callback of
> kref_put_lock() and annotate the callback with "__releases()" which
> seems to work to avoid the sparse warning. However this works if you
> don't have additional stack pointers which you need to pass to the
> release callback.
> 
> My question would be is this a known problem and a recommended way to
> avoid this sparse warning (maybe just for now)?

Hi,

I suppose that your case here can be simplified into something like:

	if (some_condition)
		take(some_lock);

	do_stuff();

	if (some_condition)
		release(some_lock);

Sparse issues the 'context imbalance' warning because, a priori,
it can't exclude that some execution will takes the lock and not
releases it (or the opposite). In some case, when do_stuff() is
very simple, sparse can see that everything is OK, but generally
it won't (some whole kernel analysis but the general case is
undecidable anyway).

The recommended way would be to write things rather like this:

	if (some_condition) {
		take(some_lock);
		do_stuff();
		release(some_lock);
	} else {
		do_stuff();
	}


The __acquires() and __releases() annotations are needed for sparse
to know that the annotated function always take or always release
some lock but won't handle conditional locks.

I hope that this is helpful to you.

-- Luc