Re: [PATCH v4 2/2] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y

[Marco Elver]

On Mon, 16 Feb 2026 at 19:00, David Laight <david.laight.linux@xxxxxxxxx> wrote:
>
> On Mon, 16 Feb 2026 15:16:23 +0100
> Marco Elver <elver@xxxxxxxxxx> wrote:
>
> > When enabling Clang's Context Analysis (aka. Thread Safety Analysis) on
> > kernel/futex/core.o (see Peter's changes at [1]), in arm64 LTO builds we
> > could see:
> >
> > | kernel/futex/core.c:982:1: warning: spinlock 'atomic ? __u.__val : q->lock_ptr' is still held at the end of function [-Wthread-safety-analysis]
> > |      982 | }
> > |          | ^
> > |    kernel/futex/core.c:976:2: note: spinlock acquired here
> > |      976 |         spin_lock(lock_ptr);
> > |          |         ^
> > | kernel/futex/core.c:982:1: warning: expecting spinlock 'q->lock_ptr' to be held at the end of function [-Wthread-safety-analysis]
> > |      982 | }
> > |          | ^
> > |    kernel/futex/core.c:966:6: note: spinlock acquired here
> > |      966 | void futex_q_lockptr_lock(struct futex_q *q)
> > |          |      ^
> > |    2 warnings generated.
> >
> > Where we have:
> >
> >       extern void futex_q_lockptr_lock(struct futex_q *q) __acquires(q->lock_ptr);
> >       ..
> >       void futex_q_lockptr_lock(struct futex_q *q)
> >       {
> >               spinlock_t *lock_ptr;
> >
> >               /*
> >                * See futex_unqueue() why lock_ptr can change.
> >                */
> >               guard(rcu)();
> >       retry:
> > >>            lock_ptr = READ_ONCE(q->lock_ptr);
>
> Did you try adding OPTIMZER_HIDE_VAR(lock_ptr) here?
> That might force the TSA logic to use 'where lock_ptr points to'
> instead of trying to allow an unlock(q->lock_ptr) (or similar)
> which is clearly entirely broken.
>
> Testing a compile with the unlock() missing ought to generate the
> warning - if not you've just confused the code enough the it stops
> caring.

OPTIMIZER_HIDE_VAR() is not appropriate as it might pessimise real
codegen which we don't want - the warning/analysis happens way earlier
in semantic analysis. But we have 'context_unsafe_alias()' which could
be used for this purpose, so 'context_unsafe_alias(lock_ptr)' after
the READ_ONCE() would work for what you intended. Except that this
function wants to be annotated with __acquires(q->lock_ptr) so the
compiler needs to be able to resolve the alias properly for this to
work.

Overall we can't really expect the compiler to see through inline asm
during semantic analysis, so we need to trick it - codegen should not
be affected for any compiler that does a reasonable job of folding
these local variable accesses.

Note, it does work for all other architectures as-is, given most don't
use inline asm for READ_ONCE().