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Wed, 19 Aug 2026 16:59:58 -0700 (PDT) Date: Wed, 19 Aug 2026 16:59:55 -0700 From: Ziyang Men To: Tejun Heo Cc: kernel-team@meta.com, Jens Axboe , Josef Bacik , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Shuah Khan , Johannes Weiner , Michal =?iso-8859-1?Q?Koutn=FD?= , Roman Gushchin , Shakeel Butt , JP Kobryn , Mykola Lysenko , linux-block@vger.kernel.org, bpf@vger.kernel.org, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/2] block: add BPF kfuncs to read blkcg io.stat Message-ID: References: <20260817214205.723267-1-ziyang.meme@gmail.com> <20260817214205.723267-2-ziyang.meme@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: Hi Tejun, On Wed, Aug 19, 2026 at 12:51:57PM -1000, Tejun Heo wrote: >Hello, > >On Wed, Aug 19, 2026 at 03:20:17PM -0700, Ziyang Men wrote: >> Yes the css_rstat_flush() is exposed but currently it is marked as SLEEPABLE, so >> it doesn't accept a RCU_PROTECTED css we proposed. This is not a problem for cpu >> ones since &cgrp->self is a trusted pointer. > >Yeah, bummer, so we get ref on cgroups but the subsys pointers are RCU, so >that means those refs are only RCU protected. I guess we can't avoid a kfunc >to get acquire a subsys css from cgroup. I wish css_rstat_flush() didn't >schedule but it does need that cond_resched(). Yeah that would be a pity. I will add kfuncs bpf_cgroup_css() and bpf_css_release() to handle the reference for the css. The bpf_blkcg_flush_stats() will be removed as well. Thanks, Ziyang > >Thanks. > >-- >tejun