From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id EF8BCC0218D for ; Wed, 29 Jan 2025 23:28:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Date:Cc:To:From:Subject:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=lru9AAVSMAW7jt9zqRkI/fcHIqiSp75XL1KohDj12kg=; b=wkf1KYIIoU+WIW 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Jan 2025 23:28:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738193318; bh=fbMzY2Z+t1cYmvMFD9g32OtU6/uJSnbJN5PrOM9htjM=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=gHXCb/OvL7vocFCjOzTdIu5QQWBp2QTWT56wpO/EjefiVNQ45gvGEAyEULPU7oFyw +JCfOWIJEnkGcQ75fMtG4LDD0sjoygmSKxn2zUdqolS0uZwbVqZ0EzJ3AJ6/cFNjDY 11n2nebIhJ2VXVC5jh5LpklQz5jp/Lpc9F55TDz35jEpr6i0P3fYfCqnaHYfGSA4At nQsKg3aRS4X5dZ13DPHpKmeWiVXroupPQX670w+d1cuxjFX11QoaSeTdtaXaVblr8g 45dQyjlyMCRYAOS38n6AGQnKohcCcKWImxb3t5jk5lVBevd3yDdZszTdUQDyL1MWRJ BIPCupLCjZPxA== Message-ID: Subject: Re: Multigrain timestamps do not work on RISC-V From: Jeff Layton To: Andreas Schwab , linux-riscv@lists.infradead.org Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Date: Wed, 29 Jan 2025 18:28:36 -0500 In-Reply-To: References: Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250129_152839_109567_3F214A33 X-CRM114-Status: GOOD ( 15.14 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Wed, 2025-01-29 at 18:06 -0500, Jeff Layton wrote: > On Wed, 2025-01-29 at 18:20 +0100, Andreas Schwab wrote: > > On Jan 29 2025, Andreas Schwab wrote: > > > > > My guess would be that something in inode_set_ctime_current is going > > > wrong. > > > > The bug is in the arch_cmpxchg macros in , they mishandle > > atomic exchange of u32 values: > > > > # fs/inode.c:2802: if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec) { > > #NO_APP > > ld a5,-96(s0) # _33, now.tv_nsec > > # fs/inode.c:2802: if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec) { > > slli a4,s2,32 #, _49, cns > > srli a4,a4,32 #, _49, _49 > > # fs/inode.c:2802: if (cns == now.tv_nsec && inode->i_ctime_sec == now.tv_sec) { > > beq a4,a5,.L1248 #, _49, _33, > > .L1205: > > addi a3,s1,120 #, __ai_ptr, inode > > .L1221: > > # fs/inode.c:2809: if (try_cmpxchg(&inode->i_ctime_nsec, &cur, now.tv_nsec)) { > > #APP > > # 2809 "fs/inode.c" 1 > > 0: lr.w a2, 0(a3) # __ret, *__ai_ptr_20 > > bne a2, a4, 1f # __ret, _49 > > > > Here the unsigned extended value of cur (a4) is compared with the sign > > extended value of inode->i_ctime_nsec (a2). They cannot match if cur > > has I_CTIME_QUERIED set (the sign bit). The lr/sc loop is exited before > > the store conditional is executed. > > > > sc.w.rl a1, a5, 0(a3) # __rc, _33, *__ai_ptr_20 > > bnez a1, 0b # __rc > > fence rw,rw > > 1: > > > > # 0 "" 2 > > #NO_APP > > sext.w a5,a2 # __ret, __ret > > > > A redundant sign extension of the current contents of inode->i_ctime_nsec. > > > > # fs/inode.c:2809: if (try_cmpxchg(&inode->i_ctime_nsec, &cur, now.tv_nsec)) { > > bne a5,s2,.L1211 #, __ret, cns, > > > > Here the sign extended value of inode->i_ctime_nsec (a5) is compared > > with the sign extended expected value (s2). They match, and try_cmpxchg > > returns true, but the store never happend. > > > > Wow, nice catch! Had me worried there for a bit! > For the record: Bit 30 is also unused. When I wrote these patches, I considered using that instead of the sign bit since I wasn't sure whether I might run into problems using the sign bit. Obviously, fixing the macro seems like the better solution, but if fixing this efficiently is a problem, then moving I_CTIME_QUERIED to bit 30 is also an option. Cheers, -- Jeff Layton _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv