Messages in this thread | | | Date | Wed, 29 Nov 2023 22:25:44 +0100 | From | Andrea Parri <> | Subject | Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command |
| |
> > As regards the Fixes: tag, I guess it boils down to what we want or we > > need to take for commit "riscv: Support membarrier private cmd". :-) > > I'm not seeing this commit in the Linux master branch, am I missing > something ?
I don't think you're missing something: I was wondering "what/where is this commit"? Sorry for the confusion.
> > FWIW, a quick git-log search confirmed that MEMBARRIER has been around > > for quite some time in the RISC-V world (though I'm not familiar with > > any of its mainstream uses): commit 1464d00b27b2 says (at least) since > > 93917ad50972 ("RISC-V: Add support for restartable sequence"). I am > > currently inclined to pick the latter commit (and check it w/ Palmer), > > but other suggestions are welcome. > > Supporting membarrier private expedited is not optional since Linux 4.14: > > see kernel/sched/core.c: > > membarrier_switch_mm(rq, prev->active_mm, next->mm); > /* > * sys_membarrier() requires an smp_mb() between setting > * rq->curr / membarrier_switch_mm() and returning to userspace. > * > * The below provides this either through switch_mm(), or in > * case 'prev->active_mm == next->mm' through > * finish_task_switch()'s mmdrop(). > */ > switch_mm_irqs_off(prev->active_mm, next->mm, next); > > Failure to provide the required barrier is a bug in the architecture's > switch_mm implementation when CONFIG_MEMBARRIER=y. > > We should probably introduce a new > Documentation/features/sched/membarrier/arch-support.txt > to clarify this requirement. > > Userspace code such as liburcu [1] heavily relies on membarrier private > expedited (when available) to speed up RCU read-side critical sections. > Various DNS servers, including BIND 9, use liburcu.
Thanks for the information.
So I should probably stick to 93917ad50972, which apparently selected CONFIG_MEMBARRIER on RISC-V, for the Fixes: tag in question.
I'll look into adding the membarrier feature you mention (as a final/ follow-up patch), unless you or someone else want to take care of it.
Andrea
| |