----- On Aug 6, 2020, at 8:13 AM, Will Deacon [email protected] wrote: > On Wed, Aug 05, 2020 at 11:22:36AM -0400, Mathieu Desnoyers wrote: >> ----- On Aug 5, 2020, at 6:59 AM, Peter Zijlstra [email protected] wrote: >> > On Tue, Aug 04, 2020 at 07:01:53PM +0200, [email protected] wrote: >> >> On Tue, Aug 04, 2020 at 10:59:33AM -0400, Mathieu Desnoyers wrote: >> >> > ----- On Aug 4, 2020, at 10:51 AM, Peter Zijlstra [email protected] >> >> > wrote: >> >> > > On Tue, Jul 28, 2020 at 12:00:10PM -0400, Mathieu Desnoyers wrote: >> >> > >> task_lock(tsk); >> >> > >> + /* >> >> > >> + * When a kthread stops operating on an address space, the loop >> >> > >> + * in membarrier_{private,global}_expedited() may not observe >> >> > >> + * that tsk->mm, and not issue an IPI. Membarrier requires a >> >> > >> + * memory barrier after accessing user-space memory, before >> >> > >> + * clearing tsk->mm. >> >> > >> + */ >> >> > >> + smp_mb(); >> >> > >> sync_mm_rss(mm); >> >> > >> local_irq_disable(); >> >> > > >> >> > > Would it make sense to put the smp_mb() inside the IRQ disable region? >> >> > >> >> > I've initially placed it right after task_lock so we could eventually >> >> > have a smp_mb__after_non_raw_spinlock or something with a much better >> >> > naming, >> >> > which would allow removing the extra barrier when it is implied by the >> >> > spinlock. >> >> >> >> Oh, right, fair enough. I'll go think about if smp_mb__after_spinlock() >> >> will work for mutexes too. >> >> >> >> It basically needs to upgrade atomic*_acquire() to smp_mb(). So that's >> >> all architectures that have their own _acquire() and an actual >> >> smp_mb__after_atomic(). >> >> >> >> Which, from the top of my head are only arm64, power and possibly riscv. >> >> And if I then git-grep smp_mb__after_spinlock, all those seem to be >> >> covered. >> >> >> >> But let me do a better audit.. >> > >> > All I could find is csky, which, afaict, defines a superfluous >> > smp_mb__after_spinlock. >> > >> > The relevant architectures are indeed power, arm64 and riscv, they all >> > have custom acquire/release and all define smp_mb__after_spinlock() >> > appropriately. >> > >> > Should we rename it to smp_mb__after_acquire() ? >> >> As discussed over IRC, smp_mb__after_atomic_acquire() would be better, >> because >> load_acquire and spin_lock have different semantic. > > Just to clarify here, are you talking about acquire on atomic RMW operations > being different to non-RMW operations, or are you talking about > atomic_read_acquire() being different to smp_load_acquire() (which I don't > think is the case, but wanted to check)?
I was referring to the two following APIs: - spin_lock() - smp_load_acquire() on x86, spin_lock() happens to be implemented with an atomic instruction, which implies a full memory barrier. However, its smp_load_acquire() does not provide a full memory barrier. Therefore, if we implement a smp_mb__after_acquire() as proposed by Peter, we could expect it to cover both APIs, which is tricky to do efficiently without adding a superfluous barrier. Hence the discussion about make its naming more specific, so it does not cover smp_load_acquire. > > We need to write this stuff down. > >> We could keep a define of smp_mb__after_spinlock to >> smp_mb__after_atomic_acquire >> to make the transition simpler. > > I'm not sure I really see the benefit of the rename, to be honest with you, > especially if smp_mb__after_spinlock() doesn't disappear at the same time. > The only reason you'd use this barrier is because the atomic is hidden away > behind a locking API, otherwise you'd just have used the full-barrier variant > of the atomic op to start with, wouldn't you? Good point. As long as we can state that smp_mb__after_spinlock applies both to raw_spinlock and non-raw spinlock (which AFAIU are mutexes on RT), I think it would suffice for our immediate needs. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
