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

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Date:   Wed, 29 Nov 2023 19:29:43 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     paulmck@...nel.org, palmer@...belt.com, paul.walmsley@...ive.com,
        aou@...s.berkeley.edu, mmaas@...gle.com, hboehm@...gle.com,
        striker@...ibm.com, charlie@...osinc.com, rehn@...osinc.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command

> > diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> > index 217fd4de61342..f63222513076d 100644
> > --- a/arch/riscv/mm/context.c
> > +++ b/arch/riscv/mm/context.c
> > @@ -323,6 +323,23 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> >   	if (unlikely(prev == next))
> >   		return;
> > +#if defined(CONFIG_MEMBARRIER) && defined(CONFIG_SMP)
> > +	/*
> > +	 * The membarrier system call requires a full memory barrier
> > +	 * after storing to rq->curr, before going back to user-space.
> > +	 *
> > +	 * Only need the full barrier when switching between processes:
> > +	 * barrier when switching from kernel to userspace is not
> > +	 * required here, given that it is implied by mmdrop(); barrier
> > +	 * when switching from userspace to kernel is not needed after
> > +	 * store to rq->curr.
> > +	 */
> > +	if (unlikely(atomic_read(&next->membarrier_state) &
> > +		     (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
> > +		      MEMBARRIER_STATE_GLOBAL_EXPEDITED)) && prev)
> > +		smp_mb();
> > +#endif
> 
> The approach looks good. Please implement it within a separate
> membarrier_arch_switch_mm() as done on powerpc.

Will do.  Thanks.

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".  :-)
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.

  Andrea