Re: [PATCH v4 12/12] xen/spinlock: support higher number of cpus

[Jürgen Groß <jgross@xxxxxxxx>]

On 29.02.24 16:46, Jan Beulich wrote:
> On 12.12.2023 10:47, Juergen Gross wrote:
> > Allow 16 bits per cpu number, which is the limit imposed by
> > spinlock_tickets_t.
> >
> > This will allow up to 65535 cpus, while increasing only the size of
> > recursive spinlocks in debug builds from 8 to 12 bytes.
> I think we want to be more conservative here, for the case of there
> being bugs: The CPU holding a lock may wrongly try to acquire it a
> 2nd time. That's the 65536th ticket then, wrapping the value.
Is this really a problem? There will be no other cpu left seeing the lock
as "free" in this case, as all others will be waiting for the head to reach
their private tail value.

> Therefore my suggestion would be to only (mention) go(ing) up to 32k.
>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   xen/common/spinlock.c      |  1 +
> >   xen/include/xen/spinlock.h | 18 +++++++++---------
> >   2 files changed, 10 insertions(+), 9 deletions(-)
> Shouldn't this also bump the upper bound of the NR_CPUS range then
> in xen/arch/Kconfig?
Fine with me, I can add another patch to the series doing that.


Juergen