Re: [PATCH v4 16/35] kref: Add context-analysis annotations

On Thu, 11 Dec 2025 at 13:26, Peter Zijlstra <[email protected]> wrote:
>
> On Thu, Nov 20, 2025 at 04:09:41PM +0100, Marco Elver wrote:
> > Mark functions that conditionally acquire the passed lock.
> >
> > Signed-off-by: Marco Elver <[email protected]>
> > ---
> >  include/linux/kref.h | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/include/linux/kref.h b/include/linux/kref.h
> > index 88e82ab1367c..9bc6abe57572 100644
> > --- a/include/linux/kref.h
> > +++ b/include/linux/kref.h
> > @@ -81,6 +81,7 @@ static inline int kref_put(struct kref *kref, void (*release)(struct kref *kref)
> >  static inline int kref_put_mutex(struct kref *kref,
> >                                void (*release)(struct kref *kref),
> >                                struct mutex *mutex)
> > +     __cond_acquires(true, mutex)
> >  {
> >       if (refcount_dec_and_mutex_lock(&kref->refcount, mutex)) {
> >               release(kref);
> > @@ -102,6 +103,7 @@ static inline int kref_put_mutex(struct kref *kref,
> >  static inline int kref_put_lock(struct kref *kref,
> >                               void (*release)(struct kref *kref),
> >                               spinlock_t *lock)
> > +     __cond_acquires(true, lock)
> >  {
> >       if (refcount_dec_and_lock(&kref->refcount, lock)) {
> >               release(kref);
> > --
> > 2.52.0.rc1.455.g30608eb744-goog
> >
>
> Note that both use the underlying refcount_dec_and_*lock() functions.
> Its a bit sad that annotation those isn't sufficient. These are inline
> functions after all, the compiler should be able to see through all that.

Wrappers will need their own annotations; for this kind of static
analysis (built-in warning diagnostic), inferring things like
__cond_acquires(true, lock) is far too complex (requires
intra-procedural control-flow analysis), and would likely be
incomplete too.

It might also be reasonable to argue that the explicit annotation is
good for documentation.

Aside: There's other static analysis tooling, like clang-analyzer that
can afford to do more complex flow-sensitive intra-procedural
analysis. But that has its own limitations, requires separate
invocation, and is pretty slow in comparison.