Re: [Xen-devel] [PATCH 2/2] smp: convert cpu_hotplug_begin into a blocking lock acquisition

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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