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Re: [Xen-devel] [PATCH 0/3] x86: fixes/improvements for scratch cpumask

On 12/02/2020 17:49, Roger Pau Monne wrote:

Hi Roger,


x86/smp: use APIC ALLBUT destination shorthand when possible

There is a more subtle problem introduced by this patch. I thought I would mention it here just in case this affect the approach you have chosen in this series.

get_cpu_maps() is used by stop_machine_run() to serialize the callers. If the latter fails to acquire the lock, it will bail out. Unfortunately, rcu_barrier() is implemented using stop_machine_run() and will be turned to pretty much a NOP if the latter fails (e.g the lock cannot be acquired).

This means that the rcu_barrier() will not do the expected job and potentially introduce unknown issues (e.g use-after-free...).

Before your patch, it would have been pretty hard to hit the problem above. After, you can race more easily with rcu_barrier() as sending IPI is pretty common.

Sadly, I don't have a suggestion yet how to fix this problem.


Julien Grall

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