svn commit: r211176 - in head/sys: amd64/amd64 i386/i386

[Attilio Rao]

2010/8/11 Kostik Belousov <kostikbel at gmail.com>:
> On Wed, Aug 11, 2010 at 04:29:21PM +0200, Attilio Rao wrote:
>> 2010/8/11 Kostik Belousov <kostikbel at gmail.com>:
>> > On Wed, Aug 11, 2010 at 01:21:46PM +0200, Attilio Rao wrote:
>> >> 2010/8/11 Kostik Belousov <kostikbel at gmail.com>:
>> >> > On Wed, Aug 11, 2010 at 10:51:27AM +0000, Attilio Rao wrote:
>> >> >> Author: attilio
>> >> >> Date: Wed Aug 11 10:51:27 2010
>> >> >> New Revision: 211176
>> >> >> URL: http://svn.freebsd.org/changeset/base/211176
>> >> >>
>> >> >> Log:
>> >> >>   IPI handlers may run generally with interrupts disabled because they
>> >> >>   are served via an interrupt gate.
>> >> >>
>> >> >>   However, that doesn't explicitly prevent preemption and thread
>> >> >>   migration thus scheduler pinning may be necessary in some handlers.
>> >> >>   Fix that.
>> >> >
>> >> > How the preemption is supposed to happen ? Aside from the explicit
>> >> > calls to mi_switch() from e.g. critical_exit().
>> >>
>> >> IIRC it should be hardclock() willing to schedule the softclock(). It
>> >> is the critical_exit() in the thread_unlock() that may trigger it
>> >> (sorry for not digging more, it was a while back that I hacked this
>> >> part, but I guess you can verify on your own).
>> >> We already have other points within the kernel that take care of
>> >> dealing with preemption/migration like lapic_handle_timer(), for
>> >> example.
>> >
>> > Right, and if the interrupts are indeed disabled, I do not see how
>> > the preemption may be triggered in the fragments like
>> >        cpu = PCPU_GET(cpuid);
>> >        cpumask = PCPU_GET(cpumask);
>>
>> I don't recall all the details and I have no time to dig now. However,
>> also spinlock_enter() does disable explicitly preemption via
>> critical_enter() after have disabled the interrupts.
>> Let me CC jhb as he implemented spinlock_enter() and may have a clue
>> about how preemption can happen with interrupts disabled.
>
> spinlock_enter() disables preemption to handle the recursive
> calls to spinlock_enter/leave, I think, to prevent switch on
> leave.

No.
Please look at how spinlock_enter() is implemented in ia32/amd64 in
order to see how it does handle recursion.

Attilio


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