linux-kernel - Re: [PATCH v2 3/4] membarrier: riscv: Provide core serializing command

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Message-ID: <ZXc82331D+qWufsL@andrea>
Date:   Mon, 11 Dec 2023 17:46:19 +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 v2 3/4] membarrier: riscv: Provide core serializing
 command

> >   	/*
> >   	 * The membarrier system call requires a full memory barrier
> >   	 * after storing to rq->curr, before going back to user-space.
> > +	 *
> > +	 * The barrier is also needed for the SYNC_CORE command when
> > +	 * switching between processes; in particular, on a transition
> > +	 * from a thread belonging to another mm to a thread belonging
> > +	 * to the mm for which a membarrier SYNC_CORE is done on CPU0:
> > +	 *
> > +	 *   - [CPU0] sets all bits in the mm icache_stale_mask.
> > +	 *
> > +	 *   - [CPU1] store to rq->curr (by the scheduler).
> > +	 *
> > +	 *   - [CPU0] loads rq->curr within membarrier and observes
> > +	 *     cpu_rq(1)->curr->mm != mm, so the IPI is skipped on
> > +	 *     CPU1; this means membarrier relies on switch_mm() to
> > +	 *     issue the sync-core.
> > +	 *
> > +	 *   - [CPU1] switch_mm() loads icache_stale_mask; if the bit
> > +	 *     is zero, switch_mm() may incorrectly skip the sync-core.
> > +	 *
> > +	 * Matches the full barrier in membarrier_private_expedited().
> 
> There are two full barriers in membarrier_private_expedited(). We
> should clearly state which one it matches, and update the associated
> barrier to state that it matches this barrier as well.

Agreed, will do.

  Andrea