From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrey Ryabinin Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed Date: Fri, 2 Nov 2018 19:19:15 +0300 Message-ID: <5a846924-e642-d9d1-4e0e-810bd4d01c26@virtuozzo.com> References: <1541015538-11382-1-git-send-email-linux@roeck-us.net> <20181031213240.zhh7dfcm47ucuyfl@pburton-laptop> <20181031220253.GA15505@roeck-us.net> <20181031233235.qbedw3pinxcuk7me@pburton-laptop> <4e2438a23d2edf03368950a72ec058d1d299c32e.camel@hammerspace.com> <20181101131846.biyilr2msonljmij@lakrids.cambridge.arm.com> <20181101145926.GE3178@hirez.programming.kicks-ass.net> <20181101163212.GF3159@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: "mark.rutland@arm.com" , "linux-kernel@vger.kernel.org" , "ralf@linux-mips.org" , "jlayton@kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "bfields@fieldses.org" , "linux-mips@linux-mips.org" , "linux@roeck-us.net" , "linux-nfs@vger.kernel.org" , "akpm@linux-foundation.org" , "will.deacon@arm.com" , "boqun.feng@gmail.com" , "paul.burton@mips.com" , "anna.schumaker@netapp.com" , "jhogan@kernel.org" , "netdev@vger.ke To: Peter Zijlstra , Trond Myklebust Return-path: In-Reply-To: <20181101163212.GF3159@hirez.programming.kicks-ass.net> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 11/01/2018 07:32 PM, Peter Zijlstra wrote: > On Thu, Nov 01, 2018 at 03:22:15PM +0000, Trond Myklebust wrote: >> On Thu, 2018-11-01 at 15:59 +0100, Peter Zijlstra wrote: >>> On Thu, Nov 01, 2018 at 01:18:46PM +0000, Mark Rutland wrote: > >>>>> My one question (and the reason why I went with cmpxchg() in the >>>>> first place) would be about the overflow behaviour for >>>>> atomic_fetch_inc() and friends. I believe those functions should >>>>> be OK on x86, so that when we overflow the counter, it behaves >>>>> like an unsigned value and wraps back around. Is that the case >>>>> for all architectures? >>>>> >>>>> i.e. are atomic_t/atomic64_t always guaranteed to behave like >>>>> u32/u64 on increment? >>>>> >>>>> I could not find any documentation that explicitly stated that >>>>> they should. >>>> >>>> Peter, Will, I understand that the atomic_t/atomic64_t ops are >>>> required to wrap per 2's-complement. IIUC the refcount code relies >>>> on this. >>>> >>>> Can you confirm? >>> >>> There is quite a bit of core code that hard assumes 2s-complement. >>> Not only for atomics but for any signed integer type. Also see the >>> kernel using -fno-strict-overflow which implies -fwrapv, which >>> defines signed overflow to behave like 2s-complement (and rids us of >>> that particular UB). >> >> Fair enough, but there have also been bugfixes to explicitly fix unsafe >> C standards assumptions for signed integers. See, for instance commit >> 5a581b367b5d "jiffies: Avoid undefined behavior from signed overflow" >> from Paul McKenney. > > Yes, I feel Paul has been to too many C/C++ committee meetings and got > properly paranoid. Which isn't always a bad thing :-) > > But for us using -fno-strict-overflow which actually defines signed > overflow, I myself am really not worried. I'm also not sure if KASAN has > been taught about this, or if it will still (incorrectly) warn about UB > for signed types. > UBSAN warns about signed overflows despite -fno-strict-overflow if gcc version is < 8. I have learned recently that UBSAN in GCC 8 ignores signed overflows if -fno-strict-overflow of fwrapv is used. $ cat signed_overflow.c #include __attribute__((noinline)) int foo(int a, int b) { return a+b; } int main(void) { int a = 0x7fffffff; int b = 2; printf("%d\n", foo(a,b)); return 0; } $ gcc-8.2.0 -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out signed_overflow.c:6:10: runtime error: signed integer overflow: 2147483647 + 2 cannot be represented in type 'int' -2147483647 $ gcc-8.2.0 -fno-strict-overflow -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out -2147483647 $ gcc-7.3.0 -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out signed_overflow.c:6:10: runtime error: signed integer overflow: 2147483647 + 2 cannot be represented in type 'int' -2147483647 $ gcc-7.3.0 -fno-strict-overflow -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out signed_overflow.c:6:10: runtime error: signed integer overflow: 2147483647 + 2 cannot be represented in type 'int' -2147483647 clang behaves the same way as GCC 8: $ clang -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out signed_overflow.c:6:10: runtime error: signed integer overflow: 2147483647 + 2 cannot be represented in type 'int' -2147483647 $ clang -fno-strict-overflow -fsanitize=signed-integer-overflow signed_overflow.c && ./a.out -2147483647 We can always just drop -fsanitize=signed-integer-overflow if it considered too noisy. Although it did catch some real bugs.