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)? 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? Will
