Date: Sun, 7 Mar 2004 17:31:14 -0800 From: Peter Wemm <peter@wemm.org> To: Alan Cox <alc@cs.rice.edu> Cc: Peter Wemm <peter@FreeBSD.org> Subject: Re: cvs commit: src/sys/i386/i386 vm_machdep.c Message-ID: <200403071731.14880.peter@wemm.org> In-Reply-To: <20040308005242.GG21071@cs.rice.edu> References: <200403080027.i280RXIC041557@repoman.freebsd.org> <20040308005242.GG21071@cs.rice.edu>
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On Sunday 07 March 2004 04:52 pm, Alan Cox wrote:
> On Sun, Mar 07, 2004 at 04:27:33PM -0800, Peter Wemm wrote:
> > peter 2004/03/07 16:27:33 PST
> >
> > FreeBSD src repository
> >
> > Modified files:
> > sys/i386/i386 vm_machdep.c
> > Log:
> > Other parts of the tree do not protect calls to kmem_free() with
> > Giant, so remove it from here. The most notable examples include
> > vm_mmap(). This removes one more Giant event from exit(2).
> >
> > Revision Changes Path
> > 1.226 +0 -2 src/sys/i386/i386/vm_machdep.c
>
> Whether this is safe depends on whether the underlying pages need to
> be unwired. An explanation can be found in uma_large_free().
> Unwiring dips into the pmap, acquiring Giant before doing so. The
> consequences would be a LOR, but not a race.
Speakling of which, Kris pasted a LOR involving this very code..
<BugMagnet> 1st 0xc07d9d40 UMA lock (UMA lock) @ vm/uma_core.c:1213
<BugMagnet> 2nd 0xc07a0540 Giant (Giant) @ vm/uma_core.c:2061
void
uma_large_free(uma_slab_t slab)
{
vsetobj((vm_offset_t)slab->us_data, kmem_object);
/*
* XXX: We get a lock order reversal if we don't have Giant:
* vm_map_remove (locks system map) -> vm_map_delete ->
* vm_map_entry_unwire -> vm_fault_unwire -> mtx_lock(&Giant)
*/
if (!mtx_owned(&Giant)) {
2061>> mtx_lock(&Giant);
page_free(slab->us_data, slab->us_size, slab->us_flags);
mtx_unlock(&Giant);
} else
page_free(slab->us_data, slab->us_size, slab->us_flags);
uma_zfree_internal(slabzone, slab, NULL, 0);
}
I'm guessing it came from a uma_reclaim()..
Meanwhile, would it be an idea to back out the Giant removal around the
kmem_free() in vm_machdep.c? Its a rarely executed code path.
I wish there was a way to keep notes about this in the source. Trying
to figure out the quirks of this stuff is painful.
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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