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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Mark Rutland , Thomas Gleixner , Kent Overstreet , Linus Torvalds , Vlastimil Babka , maged.michael@gmail.com, Neeraj Upadhyay References: <20240917143402.930114-1-boqun.feng@gmail.com> <20240917143402.930114-2-boqun.feng@gmail.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20240917143402.930114-2-boqun.feng@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: A884D120018 X-Stat-Signature: ny4xh34r44uggwgrmazxainjc5n67xty X-Rspam-User: X-HE-Tag: 1726648094-258404 X-HE-Meta: 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 HXEJ5euq HLCGdmSxD9p6yVJkYuLJmsA+4aadCOhwL+DS3yfPqOFuSbxCH1UgUdW77IFVJ6WSbHjVE/sng++9rWg5xHt4Pym+2h8yJ+4SUTvZYgkg1wzxIDrZaZYg71XAj8DKZG6dLWzpYkoeNJ891GjZxLQtR4L9+bfdtlJWyoZ7JpH8PbTRGCR/h584W+7cVL1G6HXB0JiJqfU+nN+ysii3/3xFx8RQwvolywnNNpO1bBzej4q8Yt7bP8Fxe1C0y03Jk4zhkhFRKc99QNuboXnM4KIhOgTgqxc6EZ++zmFclgxRaWyNJYVPbxSpi4aEf7T1gndNXe4CNv0MTPLqun3/WglT1taI/kslZqTy5t+DRJA85D4rdFYhN0USShe10EWNB0OFtc/KQOrAnjFnMwOq0LEAOQavuVG89dBjuOnsKGjif/h7gXY8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2024-09-17 16:33, Boqun Feng wrote: [...] > > The synchronization between readers and updaters is built around "hazard > pointer slots": a slot readers can use to store a pointer value. > > Reader side protection: > > 1. Read the value of a pointer to the target data element. > 2. Store it to a hazard pointer slot. > 3. Enforce full ordering (e.g. smp_mb()). > 4. Re-read the original pointer, reset the slot and retry if the > value changed. > 5. Otherwise, the continued existence of the target data element > is guaranteed. > > Updater side check: > > 1. Unpublish the target data element (e.g. setting the pointer > value to NULL). > 2. Enforce full ordering. > 3. Read the value from a hazard pointer slot. > 4. If the value doesn't match the target data element, then this > slot doesn't represent a reference to it. > 5. Otherwise, updater needs to re-check (step 3). Cool! I look forward to see where this is meant to be used. I would expect it to be a useful tool to speed up reference counting of things like the mm_struct and for TLB flush IPIs. On a related note, with a userspace port in mind, the membarrier(2) syscall can be useful to turn the smp_mb() in (3) from the reader side into a simple compiler barrier, assuming (2) from the updater is using membarrier. If IPIs are acceptable (or already required) for some kernel use-cases, this means a similar asymmetric fence scheme could be used to speed up readers. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com