On Wed, Aug 11, 2010 at 07:10:00PM +0200, Attilio Rao wrote: > 2010/8/11 Attilio Rao <atti...@freebsd.org>: > > 2010/8/11 Kostik Belousov <kostik...@gmail.com>: > >> 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. > > > > No. > > Please look at how spinlock_enter() is implemented in ia32/amd64 in > > order to see how it does handle recursion. > > And besides we have other patterns running with interrupts disabled > taking care of preemption as well (I think I already pointed to one, I > think you could find others easilly).
Let me rephrase the original question: how the code of the kind
a = b;
c = d;
while executed with interrupts disabled, can be a subject to the kernel
preemption ? Well, the code are slightly more involved, because
evaluation of the right part of the assignment causes rebasing against
non-default segment register on x86oids, but the detail is irrelevant.
pgpX2Ld4Ye9uL.pgp
Description: PGP signature
