Date: Thu, 25 Aug 2011 08:00:07 -0400 From: John Baldwin <jhb@freebsd.org> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-arch@freebsd.org, Hans Petter Selasky <hselasky@c2i.net> Subject: Re: skipping locks, mutex_owned, usb Message-ID: <201108250800.07467.jhb@freebsd.org> In-Reply-To: <4E563354.5020106@FreeBSD.org> References: <4E53986B.5000804@FreeBSD.org> <201108251235.15853.hselasky@c2i.net> <4E563354.5020106@FreeBSD.org>
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On Thursday, August 25, 2011 7:34:44 am Andriy Gapon wrote:
> on 25/08/2011 13:35 Hans Petter Selasky said the following:
> > On Tuesday 23 August 2011 15:11:08 John Baldwin wrote:
> >>> I. [Why] do we need this pattern?
> >>> Can the code be re-written in a smarter (if not to say proper) way?
> >>
> >
> > Hi,
> >
> >> Giant is recursive, it should just be always acquired. Also, this
> >> recursive call pattern is very non-obvious. A far more straightforward
> >> approach would be to just have:
> >
> > Unless Witness is changed, that won't work. It will lead to LOR warnings I
> > think.
> >
> > Imagine that the root function locks Giant, then another lock is locked and
> > then ukbd_poll() is called. Won't the second lock of Giant cause a LOR
> > warning?
>
> I think no.
> At least that's how I interpret the following snippet in witness_checkorder():
> /*
> * Check to see if we are recursing on a lock we already own. If
> * so, make sure that we don't mismatch exclusive and shared lock
> * acquires.
> */
> lock1 = find_instance(lock_list, lock);
> if (lock1 != NULL) {
> if ((lock1->li_flags & LI_EXCLUSIVE) != 0 &&
> (flags & LOP_EXCLUSIVE) == 0) {
> printf("shared lock of (%s) %s @ %s:%d\n",
> class->lc_name, lock->lo_name, file, line);
> printf("while exclusively locked from %s:%d\n",
> lock1->li_file, lock1->li_line);
> panic("share->excl");
> }
> if ((lock1->li_flags & LI_EXCLUSIVE) == 0 &&
> (flags & LOP_EXCLUSIVE) != 0) {
> printf("exclusive lock of (%s) %s @ %s:%d\n",
> class->lc_name, lock->lo_name, file, line);
> printf("while share locked from %s:%d\n",
> lock1->li_file, lock1->li_line);
> panic("excl->share");
> }
> return;
> }
>
> Because of the return statement we do not seem to be doing any additional order
> checking in the case of recursion.
Correct. WITNESS has never done LOR checking on trylocks or recursive
acquires since those will never block.
--
John Baldwin
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