Re: [Xen-devel] [PATCH 1/2] smp: convert the cpu maps lock into a rw lock

[Jürgen Groß <jgross@xxxxxxxx>]

On 20.02.20 09:13, Jan Beulich wrote:
> On 13.02.2020 12:32, Roger Pau Monne wrote:
> > Most users of the cpu maps just care about the maps not changing while
> > the lock is being held, but don't actually modify the maps.
> >
> > Convert the lock into a rw lock, and take the lock in read mode in
> > get_cpu_maps and in write mode in cpu_hotplug_begin. This will lower
> > the contention around the lock, since plug and unplug operations that
> > take the lock in write mode are not that common.
> >
> > Note that the read lock can be taken recursively (as it's a shared
> > lock), and hence will keep the same behavior as the previously used
> > recursive lock. As for the write lock, it's only used by CPU
> > plug/unplug operations, and the lock is never taken recursively in
> > that case.
> >
> > While there also change get_cpu_maps return type to bool.
> >
> > Reported-by: Julien Grall <julien@xxxxxxx>
> > Suggested-also-by: Jan Beulich <jbeulich@xxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> I'm afraid I can't see how offlining a CPU would now work.
> Condensed to just the relevant calls, the sequence from
> cpu_down() is
>
> cpu_hotplug_begin() (i.e. lock taken in write mode)
> stop_machine_run()
> -> get_cpu_maps() (lock unavailable to readers)
I've already pointed that out in another thread. :-)

> Other than recursive spin locks, rw locks don't currently
> have a concept of permitting in a reader when this CPU
> already holds the lock in write mode. Hence I can't see
> how the get_cpu_maps() above would now ever succeed. Am I
> missing anything, or does the patch need reverting until
> the read_trylock() got enhanced to cope with this?
I think this can be handled locally in get_cpu_maps() and
cpu_hotplug_begin() with the use of a variable holding the cpu (or
NR_CPUS) of the cpu holding the write lock. get_cpu_maps() can just
succeed in case this variable contains smp_processor_id().


Juergen

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