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

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Message-ID: <CANpmjNN+zafzhvUBBmjyy+TL1ecqJUHQNRX3bo9fBJi2nFUt=A@mail.gmail.com>
Date: Thu, 11 Dec 2025 14:54:06 +0100
From: Marco Elver <elver@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Ingo Molnar <mingo@...nel.org>, 
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Subject: Re: [PATCH v4 16/35] kref: Add context-analysis annotations

On Thu, 11 Dec 2025 at 13:26, Peter Zijlstra <peterz@...radead.org> 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 <elver@...gle.com>
> > ---
> >  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.