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.
pgpkumaJqMGOj.pgp
Description: PGP signature
