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Mon, 15 Sep 2025 06:37:39 -0400 (EDT) Date: Mon, 15 Sep 2025 20:37:34 +1000 (AEST) From: Finn Thain To: Peter Zijlstra cc: Will Deacon , Andrew Morton , Boqun Feng , Jonathan Corbet , Mark Rutland , Arnd Bergmann , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Geert Uytterhoeven , linux-m68k@vger.kernel.org Subject: Re: [RFC v2 3/3] atomic: Add alignment check to instrumented atomic operations In-Reply-To: <20250915100604.GZ3245006@noisy.programming.kicks-ass.net> Message-ID: <8247e3bd-13c2-e28c-87d8-5fd1bfed7104@linux-m68k.org> References: <20250915080054.GS3419281@noisy.programming.kicks-ass.net> <4b687706-a8f1-5f51-6e64-6eb09ae3eb5b@linux-m68k.org> <20250915100604.GZ3245006@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-m68k@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii On Mon, 15 Sep 2025, Peter Zijlstra wrote: > On Mon, Sep 15, 2025 at 07:38:52PM +1000, Finn Thain wrote: > > > > On Mon, 15 Sep 2025, Peter Zijlstra wrote: > > > > > On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote: > > > > From: Peter Zijlstra > > > > > > > > Add a Kconfig option for debug builds which logs a warning when an > > > > instrumented atomic operation takes place at some location that isn't > > > > a long word boundary. Some platforms don't trap for this. > > > > > > > > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/ > > > > --- > > > > This patch differs slightly from Peter's code which checked for natural > > > > alignment. > > > > --- > > > > include/linux/instrumented.h | 4 ++++ > > > > lib/Kconfig.debug | 10 ++++++++++ > > > > 2 files changed, 14 insertions(+) > > > > > > > > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h > > > > index 711a1f0d1a73..55f5685971a1 100644 > > > > --- a/include/linux/instrumented.h > > > > +++ b/include/linux/instrumented.h > > > > @@ -7,6 +7,7 @@ > > > > #ifndef _LINUX_INSTRUMENTED_H > > > > #define _LINUX_INSTRUMENTED_H > > > > > > > > +#include > > > > #include > > > > #include > > > > #include > > > > @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_ > > > > { > > > > kasan_check_read(v, size); > > > > kcsan_check_atomic_read(v, size); > > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1))); > > > > } > > > > > > > > /** > > > > @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size > > > > { > > > > kasan_check_write(v, size); > > > > kcsan_check_atomic_write(v, size); > > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1))); > > > > } > > > > > > > > /** > > > > @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v, > > > > { > > > > kasan_check_write(v, size); > > > > kcsan_check_atomic_read_write(v, size); > > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1))); > > > > } > > > > > > Right, so why aren't we trusting the size argument? And instead > > > mandating a possibly larger alignment? > > > > > > > It wasn't supposed to mandate a larger alignment in practice. I considered > > doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1) > > but decided that the extra overhead probably wouldn't be worthwhile, if in > > practice, no-one is doing atomic ops on shorts or chars. I will revisit > > this. > > atomic_t is aligned at 4 bytes, you're now mandating it is aligned at 8 > bytes (on LP64), this cannot be right. > > kernel/locking/qspinlock.c:xchg_tail() does xchg_relaxed(&lock->tail, > ...) which is u16. Again, you cannot mandate 8 bytes here. > OK. I will change it back to your code (i.e. mandate natural alignment). > > When you do atomic operations on atomic_t or atomic64_t, (sizeof(long) > > - 1) probably doesn't make much sense. But atomic operations get used on > > scalar types (aside from atomic_t and atomic64_t) that don't have natural > > alignment. Please refer to the other thread about this: > > https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/ > > Perhaps set ARCH_SLAB_MINALIGN ? > That's not going to help much. The 850 byte offset of task_works into struct task_struct and the 418 byte offset of exit_state in struct task_struct are already misaligned. But that's all moot, if you intended that CONFIG_DEBUG_ATOMIC should complain about any deviation from natural alignment. I still don't have any performance measurements but I'm willing to assume there's a penalty for such deviation.