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

[Julien Grall <julien@xxxxxxx>]

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

> Commit:
>
> 5500d265a2a8fa63d60c08beb549de8ec82ff7a5
> 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.

Cheers,

--
Julien Grall

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