linux-kernel - Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing command

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Date: Wed, 24 Jan 2024 22:43:36 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
	paulmck@...nel.org, corbet@....net, mmaas@...gle.com,
	hboehm@...gle.com, striker@...ibm.com, charlie@...osinc.com,
	rehn@...osinc.com, linux-riscv@...ts.infradead.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing
 command

On Wed, Jan 24, 2024 at 01:56:39PM -0500, Mathieu Desnoyers wrote:
> On 2024-01-24 13:44, Andrea Parri wrote:
> > > > +# riscv uses xRET as return from interrupt and to return to user-space.
> > > > +#
> > > > +# Given that xRET is not core serializing, we rely on FENCE.I for providing
> > > > +# core serialization:
> > > > +#
> > > > +#  - by calling sync_core_before_usermode() on return from interrupt (cf.
> > > > +#    ipi_sync_core()),
> > > > +#
> > > > +#  - via switch_mm() and sync_core_before_usermode() (respectively, for
> > > > +#    uthread->uthread and kthread->uthread transitions) to go back to
> > > > +#    user-space.
> > > 
> > > I don't quite get the meaning of the sentence above. There seems to be a
> > > missing marker before "to go back".
> > 
> > Let's see.  Without the round brackets, the last part becomes:
> > 
> >    - via switch_mm() and sync_core_before_usermode() to go back to
> >      user-space.
> > 
> > This is indeed what I meant to say.  What am I missing?
> 
> Would it still fit your intent if we say "before returning to
> user-space" rather than "to go back to user-space" ?

Yes, works for me.  Will change in v4.


> Because the switch_mm(), for instance, does not happen exactly on
> return to user-space, but rather when the scheduler switches tasks.
> Therefore, I think that stating that core serialization needs to
> happen before returning to user-space is clearer than stating that
> it happens "when" we go back to user-space.
> 
> Also, on another topic, did you find a way forward with respect of
> the different choice of words between the membarrier man page and
> documentation vs the RISC-V official semantic with respect to "core
> serializing" vs FENCE.I ?

The way forward I envision involves the continuous (iterative) discussion
/review of the respective documentation and use-cases/litmus tests/models
/etc.

AFAICS, that is not that different from discussions about smp_mb() (as in
memory-barriers.txt) vs. FENCE RW,RW (RISC-V ISA manual) - only time will
tell.

  Andrea