[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v3 4/5] evtchn: convert domain event lock to an r/w one
On 21.12.2020 18:45, Julien Grall wrote: > Hi Jan, > > On 14/12/2020 09:40, Jan Beulich wrote: >> On 11.12.2020 11:57, Julien Grall wrote: >>> On 11/12/2020 10:32, Jan Beulich wrote: >>>> On 09.12.2020 12:54, Julien Grall wrote: >>>>> 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. >>>> >>>> Well, the protection needs to be against racing changes, i.e. >>>> parallel invocations of this same function, or evtchn_close(). >>>> It is debatable whether evtchn_status() and >>>> domain_dump_evtchn_info() would better also be locked out >>>> (other read_lock() uses aren't applicable to interdomain >>>> channels). >>>> >>>>> Could you outline how a developper can find out whether he/she should >>>>> use read_lock or write_lock? >>>> >>>> I could try to, but it would again be a port type dependent >>>> model, just like for the per-channel locks. >>> >>> It is quite important to have clear locking strategy (in particular >>> rwlock) so we can make correct decision when to use read_lock or write_lock. >>> >>>> So I'd like it to >>>> be clarified first whether you aren't instead indirectly >>>> asking for these to become write_lock() >>> >>> Well, I don't understand why this is a read_lock() (even with your >>> previous explanation). I am not suggesting to switch to a write_lock(), >>> but instead asking for the reasoning behind the decision. >> >> So if what I've said in my previous reply isn't enough (including the >> argument towards using two write_lock() here), I'm struggling to >> figure what else to say. The primary goal is to exclude changes to >> the same ports. For this it is sufficient to hold just one of the two >> locks in writer mode, as the other (racing) one will acquire that >> same lock for at least reading. The question whether both need to use >> writer mode can only be decided when looking at the sites acquiring >> just one of the locks in reader mode (hence the reference to >> evtchn_status() and domain_dump_evtchn_info()) - if races with them >> are deemed to be a problem, switching to both-writers will be needed. > > I had another look at the code based on your explanation. I don't think > it is fine to allow evtchn_status() to be concurrently called with > evtchn_close(). > > evtchn_close() contains the following code: > > chn2->state = ECS_UNBOUND; > chn2->u.unbound.remote_domid = d1->domain_id; > > Where chn2 is a event channel of the remote domain (d2). Your patch will > only held the read lock for d2. > > However evtchn_status() expects the event channel state to not change > behind its back. This assumption doesn't hold for d2, and you could > possibly end up to see the new value of chn2->state after the new > chn2->u.unbound.remote_domid. > > Thanksfully, it doesn't look like chn2->u.interdomain.remote_domain > would be overwritten. Otherwise, this would be a straight dereference of > an invalid pointer. > > So I think, we need to held the write event lock for both domain. Well, okay. Three considerations though: 1) Neither evtchn_status() nor domain_dump_evtchn_info() appear to have a real need to acquire the per-domain lock. They could as well acquire the per-channel ones. (In the latter case this will then also allow inserting the so far missing process_pending_softirqs() call; it shouldn't be made with a lock held.) 2) With the double-locking changed and with 1) addressed, there's going to be almost no read_lock() left. hvm_migrate_pirqs() and do_physdev_op()'s PHYSDEVOP_eoi handling, evtchn_move_pirqs(), and hvm_dpci_msi_eoi(). While for these it may still be helpful to be possible to run in parallel, I then nevertheless wonder whether the change as a whole is still worthwhile. 3) With the per-channel double locking and with 1) addressed I can't really see the need for the double per-domain locking in evtchn_bind_interdomain() and evtchn_close(). The write lock is needed for the domain allocating a new port or freeing one. But why is there any need for holding the remote domain's lock, when its side of the channel gets guarded by the per-channel lock anyway? Granted the per-channel locks may then need acquiring a little earlier, before checking the remote channel's state. But this shouldn't be an issue. I guess I'll make addressing 1) and 3) prereq patches to this one, unless I learn of reasons why things need to remain the way they are. Jan
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