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[107.191.0.22]) by smtp.gmail.com with ESMTPSA id k142-v6sm2695525itb.0.2018.08.31.13.49.56 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 31 Aug 2018 13:49:56 -0700 (PDT) Subject: Re: [PATCH 0/3] fix blkcg offlining and destruction To: Dennis Zhou , Tejun Heo , Johannes Weiner , Josef Bacik Cc: kernel-team@fb.com, linux-block@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org References: <20180831202244.21678-1-dennisszhou@gmail.com> From: Jens Axboe Message-ID: <7b5ec635-c251-7ac3-bebd-2cfd687ffed6@kernel.dk> Date: Fri, 31 Aug 2018 14:49:54 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <20180831202244.21678-1-dennisszhou@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/31/18 2:22 PM, Dennis Zhou wrote: > Hi everyone, > > This is a split of an earlier series I sent out [1] containing the first > 3 patches with fixes from feedback. This series tackles the first > problem where blkcgs were not being destroyed. > > There is a regression in blkcg destruction where references weren't > properly put causing blkcgs to never be destroyed. Previously, blkgs > were destroyed during offlining of the blkcg. This puts back the blkcg > reference a blkg holds allowing blkcg ref to reach zero. Then, > blkcg_css_free() is called as part of the final cleanup. > > To address the problem, 0001 reverts the broken commit, 0002 delays > blkg destruction until writeback has finished, and 0003 closes the > window on a race condition between a css migration and dying, and > blkg association. This should fix the issue where blkg_get() was getting > called when a blkcg had already begun exiting. If a bio finds itself > here, it will just fall back to root. Oddly enough at one point, > blk-throttle was using policy data from and associating with potentially > different blkgs, thus how this was exposed. > > [1] https://lore.kernel.org/lkml/20180831015356.69796-1-dennisszhou@gmail.com/T > > This patchset contains the following 3 patches: > 0001-Revert-blk-throttle-fix-race-between-blkcg_bio_issue.patch > 0002-blkcg-delay-blkg-destruction-until-after-writeback-h.patch > 0003-blkcg-use-tryget-logic-when-associating-a-blkg-with-.patch > > 0001 reverts the broken commit. > 0002 delays blkg destruction until after writeback. > 0003 fixes a race condition for ongoing IO and blkcg destruction. Applied for 4.19, thanks Dennis. -- Jens Axboe