While investigating a Clang Context Analysis [1] false positive [2], I started
to dig deeper into arm64's __READ_ONCE() implementation with LTO. That rabbit
hole led me to find one critical bug with the current implementation (patch 1),
and subtle improvements that then enabled me to fix the original false positive.
Patch 1 fixes a bug where READ_ONCE() on types larger than 8 bytes (non-atomic
fallback) incorrectly qualified the pointer rather than the pointee as volatile.
This resulted in a lack of "once" semantics for large struct loads.
Patch 2 refactors the macro to use __rwonce_typeof_unqual() and
eliminates the ternary conditional.
Building on the refactor, patch 3 fixes the context analysis false positive, by
helping its alias analysis "see through" the __READ_ONCE despite the inline asm.
## Note on Alternative for Patch 3
An alternative considered for the Context Analysis fix was introducing a helper
function to redirect the pointer alias; specifically passing a pointer to
const-pointer does not invalidate an alias either (casting away the const is a
deliberate escape hatch, albeit somewhat unusual looking). This approach was
slightly more verbose, so the simpler approach was chosen for now. It is
preserved here for future reference in case we need it for something else:
static __always_inline void __set_pointer_opaque(void *const *dst, const void *val)
{
*(void **)dst = (void *)val;
}
...
__set_pointer_opaque((void *const *)&__ret, &__u.__val);
...
[1] https://docs.kernel.org/next/dev-tools/context-analysis.html
[2] https://lore.kernel.org/all/202601221040.TeM0ihff-lkp@intel.com/
---
v3:
* Comments-smithing.
* Use 'typeof(*__ret) __val'
v2:
* Add __rwonce_typeof_unqual() as fallback for old compilers.
Marco Elver (3):
arm64: Fix non-atomic __READ_ONCE() with CONFIG_LTO=y
arm64: Optimize __READ_ONCE() with CONFIG_LTO=y
arm64, compiler-context-analysis: Permit alias analysis through
__READ_ONCE() with CONFIG_LTO=y
arch/arm64/include/asm/rwonce.h | 27 ++++++++++++++++++++++-----
1 file changed, 22 insertions(+), 5 deletions(-)
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2.53.0.rc1.225.gd81095ad13-goog