Re: [Xen-devel] [PATCH 3/9] xen: sched: make locking for {insert, remove}_vcpu consistent

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/10/15 21:39, Dario Faggioli wrote:
> On Thu, 2015-10-08 at 16:20 +0100, Andrew Cooper wrote:
>> On 08/10/15 15:58, George Dunlap wrote:
>>> Generic scheduling code is called from interrupt contexts --
>>> namely,
>>> vcpu_wake()
>> There are a lot of codepaths, but I cant see one which is definitely
>> called with interrupts disables.  (OTOH, I can see several where
>> interrupts are definitely enabled).
>>
> Sorry, it's certainly me, but I don't think I understand this.
>
> AFAICT, you are saying that you fail to find in the code, situations
> the scheduler code is invoked, with interrupts already disabled, is
> this correct? In particular "definitely called with interrupt disabled"
> is what confuses me... :-/

Given a brief look at the code, I can't spot a codepath where it is
obvious that interrupts are disabled.

>
> Also (assuming what I just said is what you meant), are you referring
> "just" to schedule(), or even to other scheduler's code, which also
> disables interrupt when taking the lock(s) it needs, like, e.g.,
> vcpu_wake() as George said?

I was looking on callchains involving vcpu_wake().

>
>>> , which for the credit scheduler wants to put things on the
>>> runqueue.  Lock taken in interrupt context => interrupts must be
>>> disabled whenever taking the lock, yes?
>> Correct, which is the purpose of the ASSERT()s in the _irq() and
>> _irqsave() variants.
>>
> What ASSERT()? I don't find any assert in _spin_lock_irqsave() (which
> thing makes sense to me):

Ah yes - _irqsave() wouldn't want an assertion.

~Andrew

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