Date: Sat, 21 Aug 2004 19:15:59 -0500 From: Alan Cox <alc@cs.rice.edu> To: Brian Fundakowski Feldman <green@FreeBSD.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/vm vm_fault.c Message-ID: <20040822001559.GG9106@cs.rice.edu> In-Reply-To: <20040821234116.GC784@green.homeunix.org> References: <200408211920.i7LJKLlA088634@repoman.freebsd.org> <20040821225939.GA784@green.homeunix.org> <20040821233134.GF9106@cs.rice.edu> <20040821234116.GC784@green.homeunix.org>
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On Sat, Aug 21, 2004 at 07:41:16PM -0400, Brian Fundakowski Feldman wrote: > On Sat, Aug 21, 2004 at 06:31:34PM -0500, Alan Cox wrote: > > On Sat, Aug 21, 2004 at 06:59:39PM -0400, Brian Fundakowski Feldman wrote: > > > On Sat, Aug 21, 2004 at 07:20:21PM +0000, Alan Cox wrote: > > > > alc 2004-08-21 19:20:21 UTC > > > > > > > > FreeBSD src repository > > > > > > > > Modified files: > > > > sys/vm vm_fault.c > > > > Log: > > > > Further reduce the use of Giant by vm_fault(): Giant is held only when > > > > manipulating a vnode, e.g., calling vput(). This reduces contention for > > > > Giant during many copy-on-write faults, resulting in some additional > > > > speedup on SMPs. > > > > > > > > Note: debug_mpsafevm must be enabled for this optimization to take effect. > > > > > > This is very broken. See included first attempt at fixing it without > > > regard for actually trying to reimplement debug.mpsafenet for vnodes. > > > > > > > Can you please explain what is broken? > > #1. Lock order reversal. Giant is acquired after the map read lock. No, that is the order it has been in for months. The other functions that acquire a map lock and Giant do the same. > #2. Unlocking Giant without it being locked. This occurs in both > VM_UNLOCK_GIANT() lines I removed. I know this because my machine > panicked on the other after I removed the first. Yes, I see what happened. I was able to reproduce it. As the commit explains, I failed to condition an early unlock of Giant on debug_mpsafevm. Regards, Alan
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