Re: [Xen-devel] [PATCH 4/8] xen: add locks with timeouts for keyhandlers

From: "Jürgen Groß" <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>, "Wei Liu" <wl@xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"George Dunlap" <George.Dunlap@eu.citrix.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Ian Jackson" <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xenproject.org,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 4/8] xen: add locks with timeouts for keyhandlers
Date: Fri, 6 Mar 2020 09:15:27 +0100	[thread overview]
Message-ID: <22d6b3b2-69d1-d1d9-ba4d-a2db3d7dfb5c@suse.com> (raw)
In-Reply-To: <c9a90a26-b318-b93a-90fd-6df8292b3b55@suse.com>

On 06.03.20 09:08, Jürgen Groß wrote:
> On 05.03.20 16:25, Jan Beulich wrote:
>> On 13.02.2020 13:54, Juergen Gross wrote:
>>> Most keyhandlers are used to dump hypervisor data to the console and
>>> they are used mostly for debugging purposes. In those cases it might
>>> happen that some data structures are locked and thus are blocking the
>>> handler to access the data.
>>>
>>> In order to be able to still get some information don't use plain
>>> locking functions in the keyhandlers, but a variant of trylocks with
>>> a timeout value. This allows to wait for some time and to give up in
>>> case the lock was not obtained.
>>>
>>> Add the main infrastructure for this feature including a new runtime
>>> parameter allowing to specify the timeout value in milliseconds.
>>>
>>> Use the new locking scheme in the handlers defined in keyhandler.c.
>>
>> Personally I think trylock (as already used in some places) is the
>> way to go. Iirc others disagreed, but also didn't like the approach
>> taken here. I'm not intending to stand in the way if a majority
>> approves of this model, but I'm not going to ack these changes
>> myself.
> 
> Fair enough.
> 
> BTW, trylock is used at exactly three places: for dumping vNUMA, MSI and
> livepatch info.
> 
> And TBH: the vNUMA case is really strange, as this is a rwlock which is
> held as writer only in one place for a very brief time period when
> freeing the domain's vnuma data.
> 
> The MSI case is more complicated and looking at it in more detail I've
> realized that there is another trylock hidden in a subfunction:
> vpci_msix_arch_print(). As vpci_msix_arch_print() will drop the lock in
> the error case the interface to this function is rather weird. In
> addition the occasional softirq processing is erro prone, too, as it
> will happen only if a single domain has at least 64 MSI entries. In case
> of lots of domains with up to 63 entries watchdog timeouts can still
> happen, so I'll send a patch repairing this issue by letting
> vpci_msix_arch_print() dump only one entry and putting the loop and
> softirq handling into vpci_dump_msi().

Oh, sorry, softirqs are processed often enough, I missed one call.

Nevertheless the locking interface wants to be corrected IMO.

Juergen

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