Re: [Xen-devel] [PATCH 2/2] xen: make xen_qlock_wait() nestable

[Jan Beulich]

>>> On 01.10.18 at 11:03, <jgross@xxxxxxxx> wrote:
> On 01/10/2018 10:57, Jan Beulich wrote:
>>>>> On 01.10.18 at 09:16, <jgross@xxxxxxxx> wrote:
>>> xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
>>> of xen_qlock_kick() might be ignored in case a deeper nesting level
>>> was active right before the call of xen_poll_irq():
>>>
>>> CPU 1:                                   CPU 2:
>>> spin_lock(lock1)
>>>                                          spin_lock(lock1)
>>>                                          -> xen_qlock_wait()
>>>                                             -> xen_clear_irq_pending()
>>>                                             Interrupt happens
>>> spin_unlock(lock1)
>>> -> xen_qlock_kick(CPU 2)
>>> spin_lock_irqsave(lock2)
>>>                                          spin_lock_irqsave(lock2)
>>>                                          -> xen_qlock_wait()
>>>                                             -> xen_clear_irq_pending()
>>>                                                clears kick for lock1
>>>                                             -> xen_poll_irq()
>>> spin_unlock_irq_restore(lock2)
>>> -> xen_qlock_kick(CPU 2)
>>>                                             wakes up
>>>                                          spin_unlock_irq_restore(lock2)
>>>                                          IRET
>>>                                            resumes in xen_qlock_wait()
>>>                                            -> xen_poll_irq()
>>>                                            never wakes up
>>>
>>> The solution is to disable interrupts in xen_qlock_wait() and not to
>>> poll for the irq in case xen_qlock_wait() is called in nmi context.
>> 
>> Are precautions against NMI really worthwhile? Locks acquired both
>> in NMI context as well as outside of it are liable to deadlock anyway,
>> aren't they?
> 
> The locks don't need to be the same. A NMI-only lock tried to be
> acquired with xen_qlock_wait() for another lock having been interrupted
> by the NMI will be enough to risk the issue.

Ah, right. In which case
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan