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Re: [Xen-devel] [PATCH v2 0/6] x86: fixes/improvements for scratch cpumask

On 18.02.20 11:15, Roger Pau Monné wrote:
On Tue, Feb 18, 2020 at 08:40:58AM +0100, Jürgen Groß wrote:
On 17.02.20 19:43, Roger Pau Monne wrote:


x86/smp: use APIC ALLBUT destination shorthand when possible

Introduced a bogus usage of the scratch cpumask: it was used in a
function that could be called from interrupt context, and hence using
the scratch cpumask there is not safe. Patch #5 is a fix for that usage,
together with also preventing the usage of any per-CPU variables when
send_IPI_mask is called from #MC or #NMI context. Previous patches are
preparatory changes.

Patch #6 adds some debug infrastructure to make sure the scratch cpumask
is used in the right context, and hence should prevent further missuses.

I wonder whether it wouldn't be better to have a common percpu scratch
cpumask handling instead of introducing local ones all over the

But the scratch CPU mask already accomplishes this, it's a single
per-CPU mask allocated when the CPU is brought up.

This one, yes. There are others which are just defined like:

DEFINE_PER_CPU(cpumask_t, ...)

So basically an array of percpu cpumasks allocated when bringing up a
cpu (this spares memory as the masks wouldn't need to cover NR_CPUS
cpus), a percpu counter of the next free index and get_ and put_
functions acting in a lifo manner.

Sizing this array would be complicated IMO, and it's possible that a
failure to get a mask in certain places could lead to a panic.

The question is whether a silent double usage is better (which your
series is already addressing at least for the scratch cpumask).

This would help removing all the still existing cpumasks on the stack
and any illegal nesting would be avoided. The only remaining question
would be the size of the array.


I'm mostly worried about the size of such stack, since we would then
allow nested calls to the get_ cpumask helper. Also I'm not sure
whether we should prevent the usage in interrupt context then, in
order to try to limit the nesting as much as possible.

No, excluding interrupt context would add the need for special
purpose masks again.

I think this series is a move in the right direction, and we can add
the per-CPU stack afterwards (as the get_/put_ helpers will already be

Yes, I completely agree.


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