Message-ID: <aUAPbFJSv0alh_ix@elver.google.com>
Date: Mon, 15 Dec 2025 14:38:52 +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>,
	Will Deacon <will@...nel.org>,
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Subject: Re: [PATCH v4 06/35] cleanup: Basic compatibility with context
 analysis

On Fri, Dec 12, 2025 at 12:09PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 12, 2025 at 11:15:29AM +0100, Marco Elver wrote:
> > On Fri, 12 Dec 2025 at 10:43, Peter Zijlstra <peterz@...radead.org> wrote:
> > [..]
> > > > Correct. We're trading false negatives over false positives at this
> > > > point, just to get things to compile cleanly.
> > >
> > > Right, and this all 'works' right up to the point someone sticks a
> > > must_not_hold somewhere.
> > >
> > > > > > Better support for Linux's scoped guard design could be added in
> > > > > > future if deemed critical.
> > > > >
> > > > > I would think so, per the above I don't think this is 'right'.
> > > >
> > > > It's not sound, but we'll avoid false positives for the time being.
> > > > Maybe we can wrangle the jigsaw of macros to let it correctly acquire
> > > > and then release (via a 2nd cleanup function), it might be as simple
> > > > as marking the 'constructor' with the right __acquires(..), and then
> > > > have a 2nd __attribute__((cleanup)) variable that just does a no-op
> > > > release via __release(..) so we get the already supported pattern
> > > > above.
> > >
> > > Right, like I mentioned in my previous email; it would be lovely if at
> > > the very least __always_inline would get a *very* early pass such that
> > > the above could be resolved without inter-procedural bits. I really
> > > don't consider an __always_inline as another procedure.
> > >
> > > Because as I already noted yesterday, cleanup is now all
> > > __always_inline, and as such *should* all end up in the one function.
> > >
> > > But yes, if we can get a magical mash-up of __cleanup and __release (let
> > > it be knows as __release_on_cleanup ?) that might also work I suppose.
> > > But I vastly prefer __always_inline actually 'working' ;-)
> > 
> > The truth is that __always_inline working in this way is currently
> > infeasible. Clang and LLVM's architecture simply disallow this today:
> > the semantic analysis that -Wthread-safety does happens over the AST,
> > whereas always_inline is processed by early passes in the middle-end
> > already within LLVM's pipeline, well after semantic analysis. There's
> > a complexity budget limit for semantic analysis (type checking,
> > warnings, assorted other errors), and path-sensitive &
> > intra-procedural analysis over the plain AST is outside that budget.
> > Which is why tools like clang-analyzer exist (symbolic execution),
> > where it's possible to afford that complexity since that's not
> > something that runs for a normal compile.
> > 
> > I think I've pushed the current version of Clang's -Wthread-safety
> > already far beyond what folks were thinking is possible (a variant of
> > alias analysis), but even my healthy disregard for the impossible
> > tells me that making path-sensitive intra-procedural analysis even if
> > just for __always_inline functions is quite possibly a fool's errand.
> 
> Well, I had to propose it. Gotta push the envelope :-)
> 
> > So either we get it to work with what we have, or give up.
> 
> So I think as is, we can start. But I really do want the cleanup thing
> sorted, even if just with that __release_on_cleanup mashup or so.

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.

For <linux/cleanup.h> it needs some more thought due to extra levels of
indirection.

------ >8 ------

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))
 
diff --git a/lib/test_context-analysis.c b/lib/test_context-analysis.c
index 4612025a1065..3f72b1ab2300 100644
--- a/lib/test_context-analysis.c
+++ b/lib/test_context-analysis.c
@@ -261,6 +261,13 @@ static void __used test_seqlock_writer(struct test_seqlock_data *d)
 	write_sequnlock_irqrestore(&d->sl, flags);
 }
 
+static void __used test_seqlock_scoped(struct test_seqlock_data *d)
+{
+	scoped_seqlock_read (&d->sl, ss_lockless) {
+		(void)d->counter;
+	}
+}
+
 struct test_rwsem_data {
 	struct rw_semaphore sem;
 	int counter __guarded_by(&sem);

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