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[34.34.182.0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4ba0951fe22sm64107395e9.3.2026.11.11.05.32.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Nov 2026 05:32:17 -0800 (PST) Date: Wed, 11 Nov 2026 13:32:13 +0000 From: Vincent Donnefort To: Steven Rostedt Cc: Breno Leitao , Masami Hiramatsu , Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Steven Rostedt , kernel-team@meta.com, stable@vger.kernel.org Subject: Re: [PATCH v2] ring-buffer: Handle a reader taking the new head in rb_handle_head_page() Message-ID: References: <20261105-ring-buffer-head-moved-v2-1-05da40cebccc@debian.org> <20261105155729.0f9c7e4d@gandalf.local.home> <20261110025202.78c8c03a@fedora> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110025202.78c8c03a@fedora> On Tue, Nov 10, 2026 at 02:52:02AM -0500, Steven Rostedt wrote: > On Mon, 9 Nov 2026 09:21:32 +0000 > Vincent Donnefort wrote: > > > > The issue I think you hit was that we have two HEAD pages, when interrupts > > > come in. That is, we can get a state of: > > > > > > --> subbuf --UPDATE--> subbuf --> subbuf --HEAD--> subbuf --> > > > > How we can end-up in this situation? The cmpxch will only swap HEAD for UPDATE. > > The cpu_buffer->head_page is only updated by the reader, when it did its last read. > > Let's look at it one at a time: > > cpu_buffer->head_page > tail | > v v > +--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------+ > > Then the writer moves the HEAD page around the buffer while there are > no reads happening > > > cpu_buffer->head_page > tail | > v v > +--> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------+ > > cpu_buffer->head_page > | tail > v v > +--> subbuf ----> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf --+ > | | > +---------------------------------------------------------------------+ > > cpu_buffer->head_page > | tail > v v > +-HEAD-> subbuf ----> subbuf ----> subbuf ----> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------+ > > cpu_buffer->head_page > tail | > v v > +--> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------+ > > Now, I'll break up the update part: > > cpu_buffer->head_page > tail | > v v > +--> subbuf --UPDATE--> subbuf ----> subbuf ----> subbuf ----> subbuf --+ > | | > +-----------------------------------------------------------------------+ > > cpu_buffer->head_page > tail | > v v > +--> subbuf --UPDATE--> subbuf --HEAD--> subbuf ----> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------------+ > > An interrupt comes in and moves the head even more. > > cpu_buffer->head_page > tail | > v v > +--> subbuf --UPDATE--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------------+ > > Now the reader decides to read again and starts between the UPDATE and the HEAD > > > cpu_buffer->head_page > tail | > v v > +--> subbuf --UPDATE--> subbuf ----> subbuf --HEAD--> subbuf ----> subbuf --+ > | | > +---------------------------------------------------------------------------+ > > It finds a page with the HEAD pointer set but not the UPDATE pointer and does a swap! > > > reader_page -----+ > | > cpu_buffer->head_page | > tail | | > v v v > +--> subbuf --UPDATE--> subbuf ----> subbuf ----> subbuf --HEAD--> subbuf --+ > | | > +---------------------------------------------------------------------------+ > > Now the commit finishes and moves forward and will even hit a blank > page but the last write is now on the reader page. This breaks the > entire integrity of the ring buffer. > > Does this make sense? > > -- Steve Yes, now I spent more time on it, I believe it makes sense. I looked at what we could do and I'm trying to use a temporary RB_PAGE_HEAD | RB_PAGE_UPDATE. This link flag is then only converted to RB_PAGE_HEAD by the outer writer. I believe this would fix the race entirely... However that means the reader spins as long as the buffer writing is interrupted, which I admit is far from ideal. -- Vincent