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Fri, 1 Mar 2019 10:48:01 -0600 Received: from [172.22.218.108] (ileax41-snat.itg.ti.com [10.172.224.153]) by dlep33.itg.ti.com (8.14.3/8.13.8) with ESMTP id x21Glt8a007743; Fri, 1 Mar 2019 10:47:56 -0600 Subject: Re: [PATCH v3] cfi: fix deadloop in cfi_cmdset_0002.c do_write_buffer To: Tokunori Ikegami , "'Boris Brezillon'" , "'liujian (CE)'" CC: "'Tokunori Ikegami'" , "keescook@chromium.org" , "bbrezillon@kernel.org" , "ikegami@allied-telesis.co.jp" , "richard@nod.at" , "linux-kernel@vger.kernel.org" , "marek.vasut@gmail.com" , "linux-mtd@lists.infradead.org" , "computersforpeace@gmail.com" , "dwmw2@infradead.org" References: <1551189648-58073-1-git-send-email-liujian56@huawei.com> <016001d4cf71$865e7b60$931b7220$@gmail.com> <4F88C5DDA1E80143B232E89585ACE27D0264F137@DGGEMM528-MBX.china.huawei.com> <20190228164228.734ede80@collabora.com> <005a01d4d03e$39b0e0f0$ad12a2d0$@yahoo.co.jp> From: Vignesh Raghavendra Message-ID: <4b3bc01a-3632-5dde-f683-94744ee7179d@ti.com> Date: Fri, 1 Mar 2019 22:17:55 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <005a01d4d03e$39b0e0f0$ad12a2d0$@yahoo.co.jp> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-EXCLAIMER-MD-CONFIG: e1e8a2fd-e40a-4ac6-ac9b-f7e9cc9ee180 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [...] >>>>> In function do_write_buffer(), in the for loop, there is a case >>>>> chip_ready() returns 1 while chip_good() returns 0, so it never break >>>>> the loop. >>>>> To fix this, chip_good() is enough and it should timeout if it stay >>>>> bad for a while. >>>>> >>>>> Fixes: dfeae1073583("mtd: cfi_cmdset_0002: Change write buffer to >>>>> check correct value") >>>>> Signed-off-by: Yi Huaijie >>>>> Signed-off-by: Liu Jian >>>>> Reviewed-by: Tokunori Ikegami >>>>> --- >>>>> v2->v3: >>>>> Follow Vignesh's advice: >>>>> add one more check for check_good() even when time_after() returns >> true. >>>>> >>>>> drivers/mtd/chips/cfi_cmdset_0002.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/drivers/mtd/chips/cfi_cmdset_0002.c >>>>> b/drivers/mtd/chips/cfi_cmdset_0002.c >>>>> index 72428b6..3da2376 100644 >>>>> --- a/drivers/mtd/chips/cfi_cmdset_0002.c >>>>> +++ b/drivers/mtd/chips/cfi_cmdset_0002.c >>>>> @@ -1876,7 +1876,7 @@ static int __xipram do_write_buffer(struct >>>>> map_info *map, struct flchip *chip, >>>>> continue; >>>>> } >>>>> >>>>> - if (time_after(jiffies, timeo) && !chip_ready(map, adr)) >>>>> + if (time_after(jiffies, timeo) && !chip_good(map, adr, >>>>> datum)) >>>> >>>> Just another idea to understand easily. >>>> >>>> unsigned long now = jiffies; >>>> >>>> if (chip_good(map, adr, datum)) { >>>> xip_enable(map, chip, adr); >>>> goto op_done; >>>> } >>>> >>>> if (time_after(now, timeo) { >>>> break; >>>> } >>>> >>> >>> Thank you~. It is more easier to understand！ >>> If there are no other comments, I will send new patch again ): >> >> Except this version no longer does what Vignesh suggested. See how you >> no longer test if chip_good() is true if time_after() returns true. So, >> imagine the thread entering this function is preempted just after the >> first chip_good() test, and resumed a few ms later. time_after() will >> return true, but chip_good() might also return true, and you ignore it. > > I think that the following 3 versions will be worked for time_after() as a same result and follow the Vignesh-san suggestion. > As Boris explained above version 3 does not really follow my suggestion... Please see below > 1. Original Vignesh-san suggestion > > if (chip_good(map, adr, datum)) { > xip_enable(map, chip, adr); > goto op_done; > } > > if (time_after(jiffies, timeo)) { > /* Test chip_good() if TRUE incorrectly again so write failure by time_after() can be avoided. */ > if (chip_good(map, adr, datum)) { > xip_enable(map, chip, adr); > goto op_done; > } > break; > } > Right, here we check chip_good() once _even when time_after() is true_ to avoid _spurious_ timeout > 2. Liujian-san v3 patch > > /* Test chip_good() if FALSE correctly so write failure by time_after() can be avoided. */ > if (time_after(jiffies, timeo) && !chip_good(map, adr)) > break; > > if (chip_good(map, adr, datum)) { > xip_enable(map, chip, adr); > goto op_done; > } > This is a better version of 1 > 3. My idea > > /* Save current jiffies value before chip_good() to avoid write failure by time_after() without testing chip_good() again. */ > unsigned long now = jiffies; > > if (chip_good(map, adr, datum)) { > xip_enable(map, chip, adr); > goto op_done; > } > What if thread gets pre-empted at this point and is re-scheduled exactly after timeo jiffies have elapsed? Below check would be true and exit loop > if (time_after(now, timeo)) > break; > So, code does not check for check chip_good() after timeoout has elapsed. But chip_good() might be true at this point. Therefore leading to spurious timeout. So this version is still not right. > Note: Some brackets have been fixed from the previous mail. >