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From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.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 13:16:55 +0200	[thread overview]
Message-ID: <d2bebb20-0216-e17d-e7c3-6085ea300e26@suse.com> (raw)
In-Reply-To: <YK978wmwAZqQDEQZ@Air-de-Roger>

On 27.05.2021 13:01, Roger Pau Monné wrote:
> 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.

Well, yes, unfortunately review of earlier versions has resulted in
there being quite a few less read_lock() uses now than I had
(mistakenly) originally. There are a few worthwhile conversions,
but on the whole maybe I should indeed drop this change.

>> @@ -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.

The purpose is, as the comment says, no new event channel allocations.
Those happen under write lock, so a read-lock-based barrier is enough
here afaict.


  reply	other threads:[~2021-05-27 11:17 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
2021-05-27 11:16     ` Jan Beulich [this message]
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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