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Re: [Xen-devel] [PATCH 2/2] smp: convert cpu_hotplug_begin into a blocking lock acquisition
- To: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 19 Feb 2020 14:58:08 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 19 Feb 2020 14:58:17 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 19/02/2020 13:44, Jan Beulich wrote:
> On 19.02.2020 14:22, Roger Pau Monné wrote:
>> On Wed, Feb 19, 2020 at 01:59:51PM +0100, Jan Beulich wrote:
>>> On 13.02.2020 12:32, Roger Pau Monne wrote:
>>>> Don't allow cpu_hotplug_begin to fail by converting the trylock into a
>>>> blocking lock acquisition. Write users of the cpu_add_remove_lock are
>>>> limited to CPU plug/unplug operations, and cannot deadlock between
>>>> themselves or other users taking the lock in read mode as
>>>> cpu_add_remove_lock is always locked with interrupts enabled. There
>>>> are also no other locks taken during the plug/unplug operations.
>>> I don't think the goal was deadlock avoidance, but rather limiting
>>> of the time spent spinning while trying to acquire the lock, in
>>> favor of having the caller retry.
>> Now that the contention between read-only users is reduced as those
>> can take the lock in parallel I think it's safe to switch writers to
>> blocking mode.
> I'd agree if writers couldn't be starved by (many) readers.
XSA-114.
We fixed that vulnerability ages ago.
A writer wanting the lock will prevent further readers from taking it
before the writer drops it.
~Andrew
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