From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marco Elver Subject: Re: [PATCH 3/6] bitops: define gen_test_bit() the same way as the rest of functions Date: Tue, 7 Jun 2022 18:15:27 +0200 Message-ID: References: <20220606114908.962562-1-alexandr.lobakin@intel.com> <20220606114908.962562-4-alexandr.lobakin@intel.com> <20220607155722.44040-1-alexandr.lobakin@intel.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=KNz0hVj8TG6Qzpx0G5NqXTIz0xDB0KuRYCCZOpgg7qY=; b=a0XfGH8keS8BNyDEJAqFX9LsZ0KYzMl9tMyc2KvxF8rj4cWg0GMKO1iFWzd8v9SxGh qeIGa7tW6ioE5zIZGl5FYOjoz+A6163+aI5rfjTQtS9RxzeK+6WSGvKy5pAfcBuUr9MJ ec5u64qTYW2PApKtNVs5bfEdbrtZ0uyvJDmbDvgGnCoeD3qE5oZLj5NRvjZbEBJlshJ0 ML9wJ2nRCTgilIbJEv6Hf/wiuwZTGMiZaMU1q2szNU6afrSmDAqT459SmwiFHetNHJFE daT7TEHdSe4s7CoKN8HEXNZR4uRd6mhonRnzMVLn2mpCFZvPe1SSt7D0eQpOqJdI60iN m+iA== In-Reply-To: <20220607155722.44040-1-alexandr.lobakin@intel.com> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Alexander Lobakin Cc: Arnd Bergmann , Yury Norov , Andy Shevchenko , Richard Henderson , Matt Turner , Brian Cain , Geert Uytterhoeven , Yoshinori Sato , Rich Felker , "David S. Miller" , Kees Cook , "Peter Zijlstra (Intel)" , Borislav Petkov , Tony Luck , Greg Kroah-Hartman , linux-alpha@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger On Tue, 7 Jun 2022 at 18:05, Alexander Lobakin wrote: > > From: Marco Elver > Date: Tue, 7 Jun 2022 15:43:49 +0200 > > > On Mon, Jun 06, 2022 at 01:49PM +0200, Alexander Lobakin wrote: > > > Currently, the generic test_bit() function is defined as a one-liner > > > and in case with constant bitmaps the compiler is unable to optimize > > > it to a constant. At the same time, gen_test_and_*_bit() are being > > > optimized pretty good. > > > Define gen_test_bit() the same way as they are defined. > > > > > > Signed-off-by: Alexander Lobakin > > > --- > > > include/asm-generic/bitops/generic-non-atomic.h | 6 +++++- > > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > > > diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h > > > index 7a60adfa6e7d..202d8a3b40e1 100644 > > > --- a/include/asm-generic/bitops/generic-non-atomic.h > > > +++ b/include/asm-generic/bitops/generic-non-atomic.h > > > @@ -118,7 +118,11 @@ gen___test_and_change_bit(unsigned int nr, volatile unsigned long *addr) > > > static __always_inline int > > > gen_test_bit(unsigned int nr, const volatile unsigned long *addr) > > > { > > > - return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1))); > > > + const unsigned long *p = (const unsigned long *)addr + BIT_WORD(nr); > > > + unsigned long mask = BIT_MASK(nr); > > > + unsigned long val = *p; > > > + > > > + return !!(val & mask); > > > > Unfortunately this makes the dereference of 'addr' non-volatile, and > > effectively weakens test_bit() to the point where I'd no longer consider > > it atomic. Per atomic_bitops.txt, test_bit() is atomic. > > > > The generic version has been using a volatile access to make it atomic > > (akin to generic READ_ONCE() casting to volatile). The volatile is also > > the reason the compiler can't optimize much, because volatile forces a > > real memory access. > > Ah-ha, I see now. Thanks for catching and explaining this! > > > > > Yes, confusingly, test_bit() lives in non-atomic.h, and this had caused > > confusion before, but the decision was made that moving it will cause > > headaches for ppc so it was left alone: > > https://lore.kernel.org/all/87a78xgu8o.fsf@dja-thinkpad.axtens.net/T/#u > > > > As for how to make test_bit() more compiler-optimization friendly, I'm > > guessing that test_bit() needs some special casing where even the > > generic arch_test_bit() is different from the gen_test_bit(). > > gen_test_bit() should probably assert that whatever it is called with > > can actually be evaluated at compile-time so it is never accidentally > > used otherwise. > > I like the idea! Will do in v2. > I can move the generics and after, right below them, define > 'const_*' helpers which will mostly redirect to 'generic_*', but > for test_bit() it will be a separate function with no `volatile` > and with an assertion that the input args are constants. Be aware that there's already a "constant_test_bit()" in arch/x86/include/asm/bitops.h, which uses 2 versions of test_bit() if 'nr' is constant or not. I guess you can steer clear of that if you use "const_", but they do sound similar. > > > > I would also propose adding a comment close to the deref that test_bit() > > is atomic and the deref needs to remain volatile, so future people will > > not try to do the same optimization. > > I think that's also the reason why it's not underscored, right? Yes, the naming convention is that double-underscored ones are non-atomic so that's one clue indeed. Documentation/atomic_bitops.txt another, and unlike the other non-atomic bitops, its kernel-doc comment also does not mention "This operation is non-atomic...".