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

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Message-ID: <20251216123211.GT3707837@noisy.programming.kicks-ass.net>
Date: Tue, 16 Dec 2025 13:32:11 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Will Deacon <will@...nel.org>,
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Subject: Re: [PATCH v4 06/35] cleanup: Basic compatibility with context
 analysis

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() ?

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