Date: Sat, 14 Dec 2002 20:14:46 -0500 From: Jake Burkholder <jake@locore.ca> To: "Brian F. Feldman" <green@FreeBSD.ORG> Cc: John Baldwin <jhb@FreeBSD.ORG>, Kris Kennaway <kris@obsecurity.org>, current@FreeBSD.ORG, alpha@FreeBSD.ORG Subject: Re: UMA panic under load Message-ID: <20021214201446.F93389@locore.ca> In-Reply-To: <200212150037.gBF0bVSM014040@green.bikeshed.org>; from green@FreeBSD.ORG on Sat, Dec 14, 2002 at 07:37:31PM -0500 References: <XFMail.20021212113400.jhb@FreeBSD.org> <200212150037.gBF0bVSM014040@green.bikeshed.org>
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Apparently, On Sat, Dec 14, 2002 at 07:37:31PM -0500, Brian F. Feldman said words to the effect of; > John Baldwin <jhb@FreeBSD.org> wrote: > > > > On 12-Dec-2002 Kris Kennaway wrote: > > > I got this on an alpha tonight. It was under heavy load at the time > > > (18 simultaneous package builds had just been spawned on the machine). > > > Any ideas? > > > > > > Slab at 0xfffffc00042d3fb8, freei 2 = 0. > > > panic: Duplicate free of item 0xfffffc00042d22e0 from zone 0xfffffc0007d31800(VMSPACE) > > > > > > db_print_backtrace() at db_print_backtrace+0x18 > > > panic() at panic+0x104 > > > uma_dbg_free() at uma_dbg_free+0x170 > > > uma_zfree_arg() at uma_zfree_arg+0x150 > > > vmspace_free() at vmspace_free+0xe4 > > > swapout_procs() at swapout_procs+0x428 > > > vm_daemon() at vm_daemon+0x74 > > > fork_exit() at fork_exit+0xe0 > > > exception_return() at exception_return > > > --- root of call graph --- > > > panic > > > Stopped at Debugger+0x34: zapnot v0,#0xf,v0 <v0=0x0> > > > db> > > > > I have seen this on a couple of different arch's I think. A vmspace > > shouldn't be free'd here, it's refcount should not be that low. > > I wonder if something is free'ing the vmspace w/o dropping the refcount? > > The problem appears to be that swapout_procs() is swapping out a process > that is in the process of exiting (in exit1()) and having already > relinquished its vmspace, but has not set PRS_ZOMBIE yet (which would be > preventing the swapout). It's clearly not correct for a process in exit1() > to be swapped out, and the vmspace _needs_ to be decremented in the correct > place or resources are NEVER freed when the race is lost. P_WEXIT is set, so the process won't get swapped out. The problem is that the vmspace refcnt is 0 when swapout_procs is called, since it was decremented in exit1. The refcnt is incremented before p_flag is tested for P_WEXIT, the swapout is skipped because its found to be set, and then vmspace_free is called which decrements the refcnt to 0 and prematurely frees the vmspace. Decrementing the refcnt in exit1 breaks the normal refernce count semantics because the vmspace is not being freed then. Jake To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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