Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)

Peter Zijlstra
2022-02-10T09:33:47Z

On Wed, Feb 09, 2022 at 03:17:38PM -0500, Waiman Long wrote:
>
> On 2/9/22 14:45, Namhyung Kim wrote:
> > On Wed, Feb 9, 2022 at 11:28 AM Mathieu Desnoyers
> > <mathieu.desnoyers@efficios.com> wrote:
> > > ----- On Feb 9, 2022, at 2:22 PM, Namhyung Kim namhyung@kernel.org wrote:
> > > > I'm also concerning dynamic allocated locks in a data structure.
> > > > If we keep the info in a hash table, we should delete it when the
> > > > lock is gone. I'm not sure we have a good place to hook it up all.
> > > I was wondering about this use case as well. Can we make it mandatory to
> > > declare the lock "class" (including the name) statically, even though the
> > > lock per-se is allocated dynamically ? Then the initialization of the lock
> > > embedded within the data structure would simply refer to the lock class
> > > definition.
> > Isn't it still the same if we have static lock classes that the entry needs
> > to be deleted from the hash table when it frees the data structure?
> > I'm more concerned about free than alloc as there seems to be no
> > API to track that in a place.
>
> We may have to invent some new APIs to do that. For example,
> spin_lock_exit() can be the counterpart of spin_lock_init() and so on. Of
> course, existing kernel code have to be modified to designate the point
> after which a lock is no longer being used or is freed.

The canonical name is _destroy(). We even have mutex_destroy() except
it's usage isn't mandatory.

The easy way out is doing as lockdep does and hook into the memory
allocators and check every free'd hunk of memory for a lock. It does
hoever mean your data structure of choice needs to be able to answer: do
I have an entry in @range. Which mostly disqualifies a hash-table.

Still, I really don't think you need any of this, it's just bloat. A
very limited stack unwind for one of the two tracepoints should allow
you to find the offending lock just fine.