Date: Tue, 5 Aug 2008 16:58:30 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: Kostik Belousov <kostikbel@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: doing vfs_hash_get when vnode locked Message-ID: <Pine.GSO.4.63.0808051638410.29561@muncher.cs.uoguelph.ca> In-Reply-To: <20080805194341.GI97161@deviant.kiev.zoral.com.ua> References: <Pine.GSO.4.63.0808041657200.3482@muncher.cs.uoguelph.ca> <20080805083229.GB97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051052350.27663@muncher.cs.uoguelph.ca> <20080805153221.GG97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051242110.23305@muncher.cs.uoguelph.ca> <20080805165114.GH97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051342290.2620@muncher.cs.uoguelph.ca> <20080805194341.GI97161@deviant.kiev.zoral.com.ua>
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On Tue, 5 Aug 2008, Kostik Belousov wrote: [stuff snipped] >> Ok, I just spent a few minutes snooping around in vfs_subr.c and I think >> I see the problem. vget() has called vholdl() and then >> v_upgrade_usecount(), which has incremented the usecount and taken the >> vnode off the free list. This appears to prevent vgonel() from being >> called on it for most cases, but there is still the case in vflush() >> where the FORCECLOSE flag is set. > Yes, exactly. > [more stuff snipped] > > But, what guarantees that the vnode would not be reclaimed before/under > your vref() it ? For instance, what if the vnode is locked due to reclaim > being in progress ? > So long as I never do a vflush() with FORCECLOSE, I can't see anywhere that will vgonel() it once I have gotten it via vget(). (v_usecount incremented and not on the vnode freelist) The way I just coded it is: - the function that does the vfs_hash_get() without LK_EXCLUSIVE just fails if MNTK_UNMOUNTF is set. - my nfs_close just returns when MNTK_UNMOUNTF is set. - my nfs_unmount() doesn't set FORCECLOSE on the vflush() but instead sleeps and retries a bunch of times if vflush() fails for MNT_FORCE. - my nfs_unmount() and other code (mostly based on the vanilla FreeBSD client makes requests all fail with EINTR when MNTK_UNMOUNTF is set). I think this should work for a forced unmount, since once requests all fail and the recovery also fails, I think vflush() will work without the FORCECLOSE flag. As far as I can see, since I'm not vflush()'ng with FORCECLOSE, then nothing will vgonel() the vnode until it has been vrele()'d. (If there is a case other than vflush() with FORCECLOSE that will vgone() it when it is not on the freelist and has a v_usecount > 0, then I'll need to handle that as well, but I can't see one.) rick
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