svn commit: r202889 - head/sys/kern

[Marius Strobl]

On Thu, Jan 28, 2010 at 11:16:55AM +0100, Attilio Rao wrote:
> 2010/1/27 Marius Strobl <marius at alchemy.franken.de>:
> > On Tue, Jan 26, 2010 at 08:10:25AM +0100, Attilio Rao wrote:
> >> 2010/1/26 Rob Farmer <rfarmer at predatorlabs.net>:
> >> > On Sat, Jan 23, 2010 at 7:54 AM, Attilio Rao <attilio at freebsd.org> wrote:
> >> >> Author: attilio
> >> >> Date: Sat Jan 23 15:54:21 2010
> >> >> New Revision: 202889
> >> >> URL: http://svn.freebsd.org/changeset/base/202889
> >> >>
> >> >> Log:
> >> >> Â - Fix a race in sched_switch() of sched_4bsd.
> >> >> Â  Â In the case of the thread being on a sleepqueue or a turnstile, the
> >> >> Â  Â sched_lock was acquired (without the aid of the td_lock interface) and
> >> >> Â  Â the td_lock was dropped. This was going to break locking rules on other
> >> >> Â  Â threads willing to access to the thread (via the td_lock interface) and
> >> >> Â  Â modify his flags (allowed as long as the container lock was different
> >> >> Â  Â by the one used in sched_switch).
> >> >> Â  Â In order to prevent this situation, while sched_lock is acquired there
> >> >> Â  Â the td_lock gets blocked. [0]
> >> >> Â - Merge the ULE's internal function thread_block_switch() into the global
> >> >> Â  Â thread_lock_block() and make the former semantic as the default for
> >> >> Â  Â thread_lock_block(). This means that thread_lock_block() will not
> >> >> Â  Â disable interrupts when called (and consequently thread_unlock_block()
> >> >> Â  Â will not re-enabled them when called). This should be done manually
> >> >> Â  Â when necessary.
> >> >> Â  Â Note, however, that ULE's thread_unblock_switch() is not reaped
> >> >> Â  Â because it does reflect a difference in semantic due in ULE (the
> >> >> Â  Â td_lock may not be necessarilly still blocked_lock when calling this).
> >> >> Â  Â While asymmetric, it does describe a remarkable difference in semantic
> >> >> Â  Â that is good to keep in mind.
> >> >>
> >> >> Â [0] Reported by: Â  Â  Â Kohji Okuno
> >> >> Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â <okuno dot kohji at jp dot panasonic dot com>
> >> >> Â Tested by: Â  Â  Â  Â  Â  Â Giovanni Trematerra
> >> >> Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â <giovanni dot trematerra at gmail dot com>
> >> >> Â MFC: Â  Â  Â  Â  Â  Â  Â  Â  Â 2 weeks
> >> >>
> >> >> Modified:
> >> >> Â head/sys/kern/kern_mutex.c
> >> >> Â head/sys/kern/sched_4bsd.c
> >> >> Â head/sys/kern/sched_ule.c
> >> >
> >> > Hi,
> >> >
> >> > This commit seems to be causing me a kernel panic on sparc64 - details
> >> > are in PR 143215. Could you take a look before MFCing this?
> >>
> >> I think that the bug may be in cpu_switch() where the mutex parameter
> >> for sched_4bsd is not handled correctly.
> >> Does sparc64 support ULE? I don't think it does and I think that it
> >> simply ignores the third argument of cpu_switch() which is vital now
> >> for for sched_4bsd too (what needs to happen is to take the passed
> >> mutex and to set the TD_LOCK of old thread to be the third argument).
> >> Unluckilly, I can't do that in sparc64 asm right now, but it should
> >> not be too difficult to cope with it.
> >>
> >
> > The following patch adds handling of the mutex parameter to the
> > sparc64 cpu_switch():
> > http://people.freebsd.org/~marius/sparc64_cpu_switch_mtx.diff
> > This patch works fine with r202888. With r202889 it allows the
> > machine to boot again, however putting some load on the machine
> > causes it to issue a reset without a chance to debug. I've also
> > tried with some variations like duplicating the old cpu_switch()
> > for cpu_throw() so the altered cpu_switch() doesn't need to
> > distinguish between the to cases and only assigning old->td_lock
> > right before return but nothing made a difference. Given that
> > this leaves little room for a bug in the cpu_switch() changes I
> > suspect r202889 also breaks additional assumptions. For example
> > the sparc64 pmap code used sched_lock, does that need to change
> > to be td_lock now maybe? Is there anything else that comes to
> > your mind in this regard?
> 
> Sorry for being lame with sparc64 assembly (so that I can't make much
> more productive help here), but the required patch, sched_4bsd only,
> should simply save the extra-argument of cpu_switch() (and cpu_throw()
> is not involved, so I'm not sure what is changing there) and move in
> TD_LOCK(%oldthreadreg) when it is safe to do (just after the oldthread
> switched out completely). It doesn't even require a memory barrier.
> This patch seems a bit too big and I wonder what else it does (or I'm
> entirely wrong and that's just what I asked here), maybe adding the
> ULE support as well?

Actually it just adds old->td_lock = mtx in a non-atomic fashion
as soon as we're done with the old thread. It's "big" as I had to
reshuffle the register usage in order to preserve %i0 (old) and
%i2 (mtx) and in order to distinguish between cpu_switch() and
cpu_throw() (no mtx and old maybe be NULL in that case). As it
turns out it also works just fine, the problems I were seeing
were due to another change in that tree. Sorry for the noise.

My understanding is that for ULE, mtx should be assigned to
old->td_lock atomically, is that correct?

> 
> Said that, all the code, including MD parts should always use
> td_lock() and not doing explicit acquisitions/drops of sched_lock, if
> they want to support ULE (but probabilly even if they do not want),
> unless there is a big compelling reason (that I expect to be justified
> in comments at least).

I think the idea behind using sched_lock in the sparc64 code is
to piggyback on it for protecting the pmap and take advantage of
the fact that it's held across cpu_switch() anyway. If that's
correct it should be possible to replace it with a separate
spinlock dedicated to protecting the pmap or given that due to
the macro madness involved in mtx_{,un}lock_spin() it's hard to
properly call these from asm by calling spinlock_{enter,exit}()
directly.

Marius