Date: Wed, 23 Aug 2006 07:40:43 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Tor Egge <Tor.Egge@cvsup.no.freebsd.org> Cc: freebsd-fs@freebsd.org Subject: Re: Deadlock between nfsd and snapshots. Message-ID: <20060823044043.GA64800@deviant.kiev.zoral.com.ua> In-Reply-To: <20060822.214638.74697110.Tor.Egge@cvsup.no.freebsd.org> References: <20060821.132151.41668008.Tor.Egge@cvsup.no.freebsd.org> <20060822175540.V58720@delplex.bde.org> <20060822130743.GL56637@deviant.kiev.zoral.com.ua> <20060822.214638.74697110.Tor.Egge@cvsup.no.freebsd.org>
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--Nq2Wo0NMKNjxTN9z Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Aug 22, 2006 at 09:46:38PM +0000, Tor Egge wrote: > > 2. All places that currently set IN_ACCESS, instead would increment > > i_accessed using the atomic ops. ufs_itimes shall update i_access > > under some mutex if i_accessed is greater than zero. > > Protecting the existing i_flag and the timestamps with the vnode > interlock when the current thread only has a shared vnode lock should > be sufficient to protect against the races, removing the need for #3, > #4 and #4 below. > > What's left is avoiding setting IN_MODIFIED when it's unsafe, to > protect against the deadlock. So, I will do the following: 1. Protect both setting and reading inode times and i_flag with vnode interlock. This shall be done through all the sys/ufs/*/* code. 2. Modify ufs_itimes: > If neither IN_CHANGE nor IN_UPDATE is set then it might be unsafe to > set IN_MODIFIED since the file system might be suspended or in the > process of being suspended with the vnode sync loop in ffs_sync() > having iterated past the vnode. In other words, if IN_CHANGE or IN_UPDATE are already set, I can safely convert IN_ACCESS into IN_MOD. Otherwise, I shall implemented the algorithm below. Suspending/suspended checks need to take MNT_ILOCK. > > If the file system is suspended then IN_MODIFIED cannot be set. If > IN_MODIFIED, IN_CHANGE or IN_UPDATE is set and the file system is > suspended then something is wrong. > > If the file system is in the process of being suspended then > IN_MODIFIED can be set at the cost of triggering a restart of the > vnode sync loop in ffs_sync(). If either IN_MODIFIED, IN_CHANGE or > IN_UPDATE is already set then the vnode sync loop has not reached the > vnode, and a restart isn't needed. > > When ufs_itimes() cannot set IN_MODIFIED then it has to either risk > losing the access time update or use some mechanism to defer it (e.g. > set IN_LAZYMOD or a new flag and let process_deferred_inactive() set > IN_MODIFIED after the file system has been resumed). > BTW, shall the test for MNT_RDONLY in the ufs_itimes moved earlier ? 3. Add the process_deferred_lazymod procedure, called from ffs_snapshot before proc_deferred_inactive, that shall convert IN_LAZYMOD | IN_ACCESS into IN_MODIFIED. To be safe, the proc_def_lazymod needs vn_start_write braces. --Nq2Wo0NMKNjxTN9z Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.5 (FreeBSD) iD8DBQFE69xLC3+MBN1Mb4gRAqsrAKCEh5Tb/vYlSAXLGErJJP6AeE6H0ACcCN/O gDmX+uH3k1p+6QZWJcOY+nk= =hPmb -----END PGP SIGNATURE----- --Nq2Wo0NMKNjxTN9z--
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