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Fri, 16 Oct 2026 12:13:00 -0700 (PDT) Date: Fri, 16 Oct 2026 19:12:57 +0000 From: Igor Malovitsa To: Alexei Starovoitov Cc: bpf@vger.kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, memxor@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH bpf 1/2] bpf: Load ringbuf consumer_pos under the spinlock Message-ID: Mail-Followup-To: Igor Malovitsa , Alexei Starovoitov , bpf@vger.kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, memxor@gmail.com, linux-kernel@vger.kernel.org References: <20261016003318.529860-1-igor.malovitsa@gmail.com> <20261016003318.529860-2-igor.malovitsa@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 Content-Disposition: inline In-Reply-To: On Fri, Oct 16, 2026 at 16:14:44 +0000, Alexei Starovoitov wrote: > libbpf writes consumer_pos after every record, so this load is > a cache miss most of the time. > Today it's done before the lock. With this patch all other producers > spin on the lock while this cpu waits for it. On 64-bit too. > So "Neither changes anything on 64-bit" in the cover letter is not correct. > > imo it's not worth slowing down everyone for that. Thank you for the quick review. I do agree that a bug limited to 32-bit, requiring 4GB of traffic to reproduce is not worth the trouble. The reason I raised this patch is building a formal model and discovering there's technically a bug. If this corner case is not a concern, I can abandon this patch. The second patch is zero cost, though.