Re: [PATCH v4 06/35] cleanup: Basic compatibility with context analysis

[Marco Elver <elver@google.com>]

On Tue, Dec 16, 2025 at 01:32PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 15, 2025 at 02:38:52PM +0100, Marco Elver wrote:
> 
> > Working on rebasing this to v6.19-rc1 and saw this new scoped seqlock
> > abstraction. For that one I was able to make it work like I thought we
> > could (below). Some awkwardness is required to make it work in
> > for-loops, which only let you define variables with the same type.
> 
> > 
> > diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> > index b5563dc83aba..5162962b4b26 100644
> > --- a/include/linux/seqlock.h
> > +++ b/include/linux/seqlock.h
> > @@ -1249,6 +1249,7 @@ struct ss_tmp {
> >  };
> >  
> >  static __always_inline void __scoped_seqlock_cleanup(struct ss_tmp *sst)
> > +	__no_context_analysis
> >  {
> >  	if (sst->lock)
> >  		spin_unlock(sst->lock);
> > @@ -1278,6 +1279,7 @@ extern void __scoped_seqlock_bug(void);
> >  
> >  static __always_inline void
> >  __scoped_seqlock_next(struct ss_tmp *sst, seqlock_t *lock, enum ss_state target)
> > +	__no_context_analysis
> >  {
> >  	switch (sst->state) {
> >  	case ss_done:
> > @@ -1320,9 +1322,18 @@ __scoped_seqlock_next(struct ss_tmp *sst, seqlock_t *lock, enum ss_state target)
> >  	}
> >  }
> >  
> > +/*
> > + * Context analysis helper to release seqlock at the end of the for-scope; the
> > + * alias analysis of the compiler will recognize that the pointer @s is is an
> > + * alias to @_seqlock passed to read_seqbegin(_seqlock) below.
> > + */
> > +static __always_inline void __scoped_seqlock_cleanup_ctx(struct ss_tmp **s)
> > +	__releases_shared(*((seqlock_t **)s)) __no_context_analysis {}
> > +
> >  #define __scoped_seqlock_read(_seqlock, _target, _s)			\
> >  	for (struct ss_tmp _s __cleanup(__scoped_seqlock_cleanup) =	\
> > -	     { .state = ss_lockless, .data = read_seqbegin(_seqlock) };	\
> > +	     { .state = ss_lockless, .data = read_seqbegin(_seqlock) }, \
> > +	     *__UNIQUE_ID(ctx) __cleanup(__scoped_seqlock_cleanup_ctx) = (struct ss_tmp *)_seqlock; \
> >  	     _s.state != ss_done;					\
> >  	     __scoped_seqlock_next(&_s, _seqlock, _target))
> >  
> 
> I am ever so confused.. where is the __acquire_shared(), in read_seqbegin() ?

Ah this is just a diff on top of this v4 series. The read_seqbegin()
already had it:

	static inline unsigned read_seqbegin(const seqlock_t *sl)
		__acquires_shared(sl) __no_context_analysis
	{

> Also, why do we need this second variable with cleanup; can't the
> existing __scoped_seqlock_cleanup() get the __releases_shared()
> attribute?

The existing __scoped_seqlock_cleanup() receives &_s (struct ss_tmp *),
and we can't refer to the _seqlock from __scoped_seqlock_cleanup(). Even
if I create a member seqlock_t* ss_tmp::seqlock and initialize it with
_seqlock, the compiler can't track that the member would be an alias of
_seqlock. The function __scoped_seqlock_next() does receive _seqlock to
effectively release it executes for every loop, so there'd be a "lock
imbalance" in the compiler's eyes.

So having the direct alias (even if we cast it to make it work in the
single-statement multi-definition, the compiler doesn't care) is
required for it to work.