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From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Julien Grall <julien@xen.org>,
	Wei Liu <wl@xen.org>, Stefano Stabellini <sstabellini@kernel.org>,
	Kevin Tian <kevin.tian@intel.com>
Subject: Re: [PATCH v5 2/6] evtchn: convert domain event lock to an r/w one
Date: Thu, 27 May 2021 13:01:07 +0200	[thread overview]
Message-ID: <YK978wmwAZqQDEQZ@Air-de-Roger> (raw)
In-Reply-To: <5f5fc6a7-6e27-8275-0f05-11ba5454156a@suse.com>

On Wed, Jan 27, 2021 at 09:16:07AM +0100, Jan Beulich wrote:
> Especially for the use in evtchn_move_pirqs() (called when moving a vCPU
> across pCPU-s) and the ones in EOI handling in PCI pass-through code,
> serializing perhaps an entire domain isn't helpful when no state (which
> isn't e.g. further protected by the per-channel lock) changes.

I'm unsure this move is good from a performance PoV, as the operations
switched to use the lock in read mode is a very small subset, and then
the remaining operations get a performance penalty when compared to
using a plain spin lock.

> Unfortunately this implies dropping of lock profiling for this lock,
> until r/w locks may get enabled for such functionality.
> While ->notify_vcpu_id is now meant to be consistently updated with the
> per-channel lock held, an extension applies to ECS_PIRQ: The field is
> also guaranteed to not change with the per-domain event lock held for
> writing. Therefore the link_pirq_port() call from evtchn_bind_pirq()
> could in principle be moved out of the per-channel locked regions, but
> this further code churn didn't seem worth it.
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> @@ -1510,9 +1509,10 @@ int evtchn_destroy(struct domain *d)
>  {
>      unsigned int i;
> -    /* After this barrier no new event-channel allocations can occur. */
> +    /* After this kind-of-barrier no new event-channel allocations can occur. */
>      BUG_ON(!d->is_dying);
> -    spin_barrier(&d->event_lock);
> +    read_lock(&d->event_lock);
> +    read_unlock(&d->event_lock);

Don't you want to use write mode here to assure there are no read
users that have taken the lock before is_dying has been set, and thus
could make wrong assumptions?

As I understand the point of the barrier here is to ensure there are
no lockers carrier over from before is_dying has been set.

Thanks, Roger.

  reply	other threads:[~2021-05-27 11:01 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-27  8:13 [PATCH v5 0/6] evtchn: (not so) recent XSAs follow-on Jan Beulich
2021-01-27  8:15 ` [PATCH v5 1/6] evtchn: use per-channel lock where possible Jan Beulich
2021-01-27  8:16 ` [PATCH v5 2/6] evtchn: convert domain event lock to an r/w one Jan Beulich
2021-05-27 11:01   ` Roger Pau Monné [this message]
2021-05-27 11:16     ` Jan Beulich
2022-07-07 18:00   ` Julien Grall
2021-01-27  8:16 ` [PATCH v5 3/6] evtchn: slightly defer lock acquire where possible Jan Beulich
2021-01-27  8:16 ` [PATCH v5 4/6] evtchn: add helper for port_is_valid() + evtchn_from_port() Jan Beulich
2021-01-27  8:17 ` [PATCH v5 5/6] evtchn: type adjustments Jan Beulich
2021-01-27  8:17 ` [PATCH v5 6/6] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() Jan Beulich
2021-04-21 15:23 ` Ping: [PATCH v5 0/6] evtchn: (not so) recent XSAs follow-on Jan Beulich
2021-04-21 15:56   ` Julien Grall
2021-04-22  8:53     ` Jan Beulich
2021-05-14 15:29       ` Roger Pau Monné
2021-05-17  7:15         ` Jan Beulich

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