From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932388AbeCMR3p (ORCPT ); Tue, 13 Mar 2018 13:29:45 -0400 Received: from mail-yw0-f193.google.com ([209.85.161.193]:39132 "EHLO mail-yw0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751626AbeCMR3l (ORCPT ); Tue, 13 Mar 2018 13:29:41 -0400 X-Google-Smtp-Source: AG47ELtXhO6wsCjrR0meGg5hVlL1biTHdRSl4wsTn7pOfT6TDrmrwNKBfikB5eB2zXnNxvwCwknCgZ8Ni6DeZ5IOlq0= MIME-Version: 1.0 In-Reply-To: References: <201803122219.vHl3IwRo%fengguang.wu@intel.com> <20180313104652.GK4043@hirez.programming.kicks-ass.net> From: Christopher Li Date: Tue, 13 Mar 2018 10:29:40 -0700 X-Google-Sender-Auth: ZglCEQh5n-IFE3B0E7xZeGVlhok Message-ID: Subject: Re: [tip:locking/core 9/11] include/asm-generic/atomic-instrumented.h:288:24: sparse: cast truncates bits from constant value (100 becomes 0) To: Dmitry Vyukov Cc: Peter Zijlstra , Linux-Sparse , kbuild test robot , kbuild-all@01.org, LKML , tipbuild@zytor.com, Ingo Molnar Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 13, 2018 at 4:08 AM, Dmitry Vyukov wrote: > On Tue, Mar 13, 2018 at 1:46 PM, Peter Zijlstra wrote: >> >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 283 static __always_inline unsigned long >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 284 cmpxchg_size(volatile void *ptr, unsigned long old, unsigned long new, int size) >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 285 { >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 286 switch (size) { >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 287 case 1: >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 @288 return arch_cmpxchg((u8 *)ptr, (u8)old, (u8)new); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 289 case 2: >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 290 return arch_cmpxchg((u16 *)ptr, (u16)old, (u16)new); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 291 case 4: >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 292 return arch_cmpxchg((u32 *)ptr, (u32)old, (u32)new); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 293 case 8: >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 294 BUILD_BUG_ON(sizeof(unsigned long) != 8); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 295 return arch_cmpxchg((u64 *)ptr, (u64)old, (u64)new); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 296 } >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 297 BUILD_BUG(); >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 298 return 0; >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 299 } >>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 300 >> >>> It seems that this is due to this guy: >>> >>> static __always_inline int trylock_clear_pending(struct qspinlock *lock) >>> { >>> struct __qspinlock *l = (void *)lock; >>> >>> return !READ_ONCE(l->locked) && >>> (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL, >>> _Q_LOCKED_VAL) == _Q_PENDING_VAL); >>> } >>> >>> _Q_PENDING_VAL is 0x100. However, locked_pending is 2 bytes. So it >>> seems that compiler checks all switch cases, this inevitably will lead >>> to such warnings. So you are saying cmpxchg_size() was call with a size==2, however sparse did not do the constant propagation or evaluation properly. So that it did not eliminate the other branch of size==1 case statement. Can you do a "sparse -E" on the original file, then save the result to a new file. That will take care of the pre-processing. Then run the sparse on the pre-processed file to get a smaller test case? Having a smaller test case would make it easier to reproduce what exactly IR was issued during that warning. >>> Off the top of my head I can think of the following solution: >>> >>> switch (size) { >>> case 1: >>> return arch_cmpxchg((u8 *)ptr, (u8)(old * (size != >>> 1)), (u8)(new * (size != 1))); >>> case 2: >>> return arch_cmpxchg((u16 *)ptr, (u16)(old * (size != >>> 2)), (u16)(new * (size != 2))); >>> >>> But it's too ugly. >> >> Yes agreed, that's horrendous. Let's not do that. If it is a sparse problem, let's try to fix this sparse. Chris