From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
George Dunlap <George.Dunlap@eu.citrix.com>,
Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH v3 4/5] evtchn: convert domain event lock to an r/w one
Date: Wed, 9 Dec 2020 11:54:13 +0000 [thread overview]
Message-ID: <074be931-54b0-1b0f-72d8-5bd577884814@xen.org> (raw)
In-Reply-To: <a333387e-f9e5-7051-569a-1a9a37da53ca@suse.com>
Hi Jan,
On 23/11/2020 13:29, Jan Beulich wrote:
> @@ -620,7 +620,7 @@ int evtchn_close(struct domain *d1, int
> long rc = 0;
>
> again:
> - spin_lock(&d1->event_lock);
> + write_lock(&d1->event_lock);
>
> if ( !port_is_valid(d1, port1) )
> {
> @@ -690,13 +690,11 @@ int evtchn_close(struct domain *d1, int
> BUG();
>
> if ( d1 < d2 )
> - {
> - spin_lock(&d2->event_lock);
> - }
> + read_lock(&d2->event_lock);
This change made me realized that I don't quite understand how the
rwlock is meant to work for event_lock. I was actually expecting this to
be a write_lock() given there are state changed in the d2 events.
Could you outline how a developper can find out whether he/she should
use read_lock or write_lock?
[...]
> --- a/xen/common/rwlock.c
> +++ b/xen/common/rwlock.c
> @@ -102,6 +102,14 @@ void queue_write_lock_slowpath(rwlock_t
> spin_unlock(&lock->lock);
> }
>
> +void _rw_barrier(rwlock_t *lock)
> +{
> + check_barrier(&lock->lock.debug);
> + smp_mb();
> + while ( _rw_is_locked(lock) )
> + arch_lock_relax();
> + smp_mb();
> +}
As I pointed out when this implementation was first proposed (see [1]),
there is a risk that the loop will never exit.
I think the following implementation would be better (although it is ugly):
write_lock();
/* do nothing */
write_unlock();
This will act as a barrier between lock held before and after the call.
As an aside, I think the introduction of rw_barrier() deserve to be a in
separate patch to help the review.
Cheers,
--
Julien Grall
next prev parent reply other threads:[~2020-12-09 11:54 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-23 13:26 [PATCH v3 0/5] evtchn: (not so) recent XSAs follow-on Jan Beulich
2020-11-23 13:28 ` [PATCH v3 1/5] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() Jan Beulich
2020-12-02 19:03 ` Julien Grall
2020-12-03 9:46 ` Jan Beulich
2020-12-09 9:53 ` Julien Grall
2020-12-09 14:24 ` Jan Beulich
2020-11-23 13:28 ` [PATCH v3 2/5] evtchn: avoid access tearing for ->virq_to_evtchn[] accesses Jan Beulich
2020-12-02 21:14 ` Julien Grall
2020-11-23 13:28 ` [PATCH v3 3/5] evtchn: convert vIRQ lock to an r/w one Jan Beulich
2020-12-09 11:16 ` Julien Grall
2020-11-23 13:29 ` [PATCH v3 4/5] evtchn: convert domain event " Jan Beulich
2020-12-09 11:54 ` Julien Grall [this message]
2020-12-11 10:32 ` Jan Beulich
2020-12-11 10:57 ` Julien Grall
2020-12-14 9:40 ` Jan Beulich
2020-12-21 17:45 ` Julien Grall
2020-12-22 9:46 ` Jan Beulich
2020-12-23 11:22 ` Julien Grall
2020-12-23 12:57 ` Jan Beulich
2020-12-23 13:19 ` Julien Grall
2020-12-23 13:36 ` Jan Beulich
2020-11-23 13:30 ` [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held Jan Beulich
2020-11-30 10:39 ` Isaila Alexandru
2020-12-02 21:10 ` Julien Grall
2020-12-03 10:09 ` Jan Beulich
2020-12-03 14:40 ` Tamas K Lengyel
2020-12-04 11:28 ` Julien Grall
2020-12-04 11:48 ` Jan Beulich
2020-12-04 11:51 ` Julien Grall
2020-12-04 12:01 ` Jan Beulich
2020-12-04 15:09 ` Julien Grall
2020-12-07 8:02 ` Jan Beulich
2020-12-07 17:22 ` Julien Grall
2020-12-04 15:21 ` Tamas K Lengyel
2020-12-04 15:29 ` Julien Grall
2020-12-04 19:15 ` Tamas K Lengyel
2020-12-04 19:22 ` Julien Grall
2020-12-04 21:23 ` Tamas K Lengyel
2020-12-07 15:28 ` Jan Beulich
2020-12-07 17:30 ` Julien Grall
2020-12-07 17:35 ` Tamas K Lengyel
2020-12-23 13:12 ` Jan Beulich
2020-12-23 13:33 ` Julien Grall
2020-12-23 13:41 ` Jan Beulich
2020-12-23 14:44 ` Julien Grall
2020-12-23 14:56 ` Jan Beulich
2020-12-23 15:08 ` Julien Grall
2020-12-23 15:15 ` Tamas K Lengyel
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