Date: Sun, 05 Apr 2015 12:45:00 -0500 From: Alan Cox <alc@rice.edu> To: Gleb Smirnoff <glebius@FreeBSD.org>, David Wolfskill <david@catwhisker.org>, current@freebsd.org Cc: alc@FreeBSD.org Subject: Re: panic: Lock vm object not exclusively locked @ /usr/src/sys/vm/vm_page.c:2637 Message-ID: <5521749C.4020408@rice.edu> In-Reply-To: <20150405154721.GO64665@FreeBSD.org> References: <20150405133758.GA40261@albert.catwhisker.org> <20150405154721.GO64665@FreeBSD.org>
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On 04/05/2015 10:47, Gleb Smirnoff wrote: > On Sun, Apr 05, 2015 at 06:37:58AM -0700, David Wolfskill wrote: > D> It ocurred rather late in the transition to multi-user mode, but > D> prior to starting xdm (on my laptop). > D>=20 > D> Previous (working) head/i386 for this machine was r281074. > D>=20 > D> Here's the first bit of the crashinfo (yes, I have a crash dump): > D>=20 > D> g1-254.catwhisker.org dumped core - see /var/crash/vmcore.3 > D>=20 > D> Sun Apr 5 06:18:44 PDT 2015 > D>=20 > D> FreeBSD g1-254.catwhisker.org 11.0-CURRENT FreeBSD 11.0-CURRENT #156= 1 r281106M/281106:1100067: Sun Apr 5 06:01:06 PDT 2015 root@g1-254.= catwhisker.org:/common/S4/obj/usr/src/sys/CANARY i386 > D>=20 > D> panic: Lock vm object not exclusively locked @ /usr/src/sys/vm/vm_pa= ge.c:2637 > > This is r281079. > > Since vm_page_advise() may call vm_page_dirty() in the MADV_DONTNEED ca= se, > the assertion is valid. So, looks like vm_fault_dontneed() needs W-lock= on > the first_object. > Actually, what I forgot was that vm_page_advise(MADV_FREE) clears the page's dirty field, and that is why an exclusive lock is asserted. As explained in vm_page.h, the pmap is allowed to set the dirty field to all ones without any locking. Moreover, the new "fast" path in vm_fault() sets the dirty field with only a read lock held.=20 vm_page_advise(MADV_DONTNEED) isn't really any different from the fast pa= th. Need to think a bit ... =20
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