On Wed, Aug 11, 2010 at 05:46:46PM +0300, Kostik Belousov wrote: > On Wed, Aug 11, 2010 at 04:29:21PM +0200, Attilio Rao wrote: > > 2010/8/11 Kostik Belousov <kostik...@gmail.com>: > > > On Wed, Aug 11, 2010 at 01:21:46PM +0200, Attilio Rao wrote: > > >> 2010/8/11 Kostik Belousov <kostik...@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. > > I do understand the reason for your change. do _not_ understand. sorry.
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