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Thu, 22 Oct 2020 08:49:05 +0000 (UTC) Received: from [10.36.113.152] (ovpn-113-152.ams2.redhat.com [10.36.113.152]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8980C60C04; Thu, 22 Oct 2020 08:49:00 +0000 (UTC) Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c" To: David Laight , Greg KH , Al Viro , Nick Desaulniers Cc: Christoph Hellwig , "kernel-team@android.com" , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" References: <20200925045146.1283714-1-hch@lst.de> <20200925045146.1283714-3-hch@lst.de> <20201021161301.GA1196312@kroah.com> <20201021233914.GR3576660@ZenIV.linux.org.uk> <20201022082654.GA1477657@kroah.com> <80a2e5fa-718a-8433-1ab0-dd5b3e3b5416@redhat.com> <5d2ecb24db1e415b8ff88261435386ec@AcuMS.aculab.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Thu, 22 Oct 2020 10:48:59 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <5d2ecb24db1e415b8ff88261435386ec@AcuMS.aculab.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 22.10.20 10:40, David Laight wrote: > From: David Hildenbrand >> Sent: 22 October 2020 09:35 >> >> On 22.10.20 10:26, Greg KH wrote: >>> On Thu, Oct 22, 2020 at 12:39:14AM +0100, Al Viro wrote: >>>> On Wed, Oct 21, 2020 at 06:13:01PM +0200, Greg KH wrote: >>>>> On Fri, Sep 25, 2020 at 06:51:39AM +0200, Christoph Hellwig wrote: >>>>>> From: David Laight >>>>>> >>>>>> This lets the compiler inline it into import_iovec() generating >>>>>> much better code. >>>>>> >>>>>> Signed-off-by: David Laight >>>>>> Signed-off-by: Christoph Hellwig >>>>>> --- >>>>>> fs/read_write.c | 179 ------------------------------------------------ >>>>>> lib/iov_iter.c | 176 +++++++++++++++++++++++++++++++++++++++++++++++ >>>>>> 2 files changed, 176 insertions(+), 179 deletions(-) >>>>> >>>>> Strangely, this commit causes a regression in Linus's tree right now. >>>>> >>>>> I can't really figure out what the regression is, only that this commit >>>>> triggers a "large Android system binary" from working properly. There's >>>>> no kernel log messages anywhere, and I don't have any way to strace the >>>>> thing in the testing framework, so any hints that people can provide >>>>> would be most appreciated. >>>> >>>> It's a pure move - modulo changed line breaks in the argument lists >>>> the functions involved are identical before and after that (just checked >>>> that directly, by checking out the trees before and after, extracting two >>>> functions in question from fs/read_write.c and lib/iov_iter.c (before and >>>> after, resp.) and checking the diff between those. >>>> >>>> How certain is your bisection? >>> >>> The bisection is very reproducable. >>> >>> But, this looks now to be a compiler bug. I'm using the latest version >>> of clang and if I put "noinline" at the front of the function, >>> everything works. >> >> Well, the compiler can do more invasive optimizations when inlining. If >> you have buggy code that relies on some unspecified behavior, inlining >> can change the behavior ... but going over that code, there isn't too >> much action going on. At least nothing screamed at me. > > Apart from all the optimisations that get rid off the 'pass be reference' > parameters and strange conditional tests. > Plenty of scope for the compiler getting it wrong. > But nothing even vaguely illegal. Not the first time that people blame the compiler to then figure out that something else is wrong ... but maybe this time is different :) -- Thanks, David / dhildenb