[PATCH 2/2] membarrier: riscv: Provide core serializing command
Andrea Parri
parri.andrea at gmail.com
Wed Nov 29 13:25:44 PST 2023
> > 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
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