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[34.34.182.0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4bb4e251c26sm63991125e9.1.2026.11.12.03.51.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Nov 2026 03:51:31 -0800 (PST) Date: Thu, 12 Nov 2026 11:51:28 +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> <20261112061423.59e02261@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: <20261112061423.59e02261@fedora> On Thu, Nov 12, 2026 at 06:14:23AM -0500, Steven Rostedt wrote: > On Wed, 11 Nov 2026 13:32:13 +0000 > Vincent Donnefort wrote: > > > 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. > > But it is only interrupted by interrupts ;-) which should be fast. > > No different than waiting for a spin lock whose owner got interrupted > by an interrupt. > > -- Steve Yes hopefully it shouldn't be too terrible. But, as we disable interrupts on the CPU doing the read, it's sort of propagating the latency from one CPU to the other, which I believe wasn't the case so far, since an interrupt could "release" the reader by setting HEAD. Perhaps I could propagate a -EAGAIN back to ring_buffer_consume to temporarily release rb_reader_lock() and try again there? But that would be a bit intrusive as most of the functions are returning a ring_buffer_event * and not an int. The attempt is here [1]. Sashiko had something to say but I haven't processed yet how bad it is. [1] https://lore.kernel.org/all/20261111160624.1294946-1-vdonnefort@google.com/ -- Vincent