On Thu, Mar 07, 2024 at 08:53:05AM -0500, Mathieu Desnoyers wrote: > On 2024-03-06 22:37, Paul E. McKenney wrote: > > On Wed, Mar 06, 2024 at 10:06:21PM -0500, Mathieu Desnoyers wrote: > [...] > > > > > As far as the WRITE_ONCE(x, READ_ONCE(x) + 1) pattern > > > is concerned, the only valid use-case I can think of is > > > split counters or RCU implementations where there is a > > > single updater doing the increment, and one or more > > > concurrent reader threads that need to snapshot a > > > consistent value with READ_ONCE(). > > > [...] > > > > So what would you use that pattern for? > > > > One possibility is a per-CPU statistical counter in userspace on a > > fastpath, in cases where losing the occasional count is OK. Then learning > > your CPU (and possibly being immediately migrated to some other CPU), > > READ_ONCE() of the count, increment, and WRITE_ONCE() might (or might not) > > make sense. > > > > I suppose the same in the kernel if there was a fastpath so extreme you > > could not afford to disable preemption. > > > > At best, very niche. > > > > Or am I suffering a failure of imagination yet again? ;-) > > The (niche) use-cases I have in mind are split-counters and RCU > grace period tracking, where precise counters totals are needed > (no lost count). > > In the kernel, this could be: Thank you for looking into this! > - A per-cpu counter, each counter incremented from thread context with > preemption disabled (single updater per counter), read concurrently by > other threads. WRITE_ONCE/READ_ONCE is useful to make sure there > is no store/load tearing there. Atomics on the update would be stronger > than necessary on the increment fast-path. But if preemption is disabled, the updater can read the value without READ_ONCE() without risk of concurrent update. Or are you concerned about interrupt handlers? This would have to be a read from the interrupt handler, given that an updated from the interrupt handler could result in a lost count. > - A per-thread counter (e.g. within task_struct), only incremented by the > single thread, read by various threads concurrently. Ditto. > - A counter which increment happens to be already protected by a lock, read > by various threads without taking the lock. (technically doable, but > I'm not sure I see a relevant use-case for it) In that case, the lock would exclude concurrent updates, so the lock holder would not need READ_ONCE(), correct? > In user-space: > > - The "per-cpu" counter would have to use rseq for increments to prevent > inopportune migrations, which needs to be implemented in assembler anyway. > The counter reads would have to use READ_ONCE(). Fair enough! > - The per-thread counter (Thread-Local Storage) incremented by a single > thread, read by various threads concurrently, is a good target > for WRITE_ONCE()/READ_ONCE() pairing. This is actually what we do in > various liburcu implementations which track read-side critical sections > per-thread. Agreed, but do any of these use WRITE_ONCE(x, READ_ONCE(x) + 1) or similar? > - Same as for the kernel, a counter increment protected by a lock which > needs to be read from various threads concurrently without taking > the lock could be a valid use-case, though I fail to see how it is > useful due to lack of imagination on my part. ;-) In RCU, we have "WRITE_ONCE(*sp, *sp + 1)" for this use case, though here we have the WRITE_ONCE() but not the READ_ONCE() because we hold the lock excluding any other updates. > I'm possibly missing other use-cases, but those come to mind as not > involving racy counter increments. > > I agree that use-cases are so niche that we probably do _not_ want to > introduce ADD_SHARED() for that purpose in a common header file, > because I suspect that it would then become misused in plenty of > scenarios where the updates are actually racy and would be better > served by atomics or local-atomics. I agree that unless or until we have a reasonable number of use cases, we should open-code it. With a big fat comment explaining how it works. ;-) Thanx, Paul
