Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command

[Andrea Parri]

> > 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