Date: Tue, 11 Oct 2011 13:22:35 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-9@freebsd.org Subject: svn commit: r226251 - stable/9/sys/kern Message-ID: <201110111322.p9BDMZR1016900@svn.freebsd.org>
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Author: kib Date: Tue Oct 11 13:22:35 2011 New Revision: 226251 URL: http://svn.freebsd.org/changeset/base/226251 Log: MFC r226022: Move parts of the commit log for r166167, where Tor explained the interaction between vnode locks and vfs_busy(), into comment. Approved by: re (bz) Modified: stable/9/sys/kern/vfs_subr.c Directory Properties: stable/9/sys/ (props changed) stable/9/sys/amd64/include/xen/ (props changed) stable/9/sys/boot/ (props changed) stable/9/sys/boot/i386/efi/ (props changed) stable/9/sys/boot/ia64/efi/ (props changed) stable/9/sys/boot/ia64/ski/ (props changed) stable/9/sys/boot/powerpc/boot1.chrp/ (props changed) stable/9/sys/boot/powerpc/ofw/ (props changed) stable/9/sys/cddl/contrib/opensolaris/ (props changed) stable/9/sys/conf/ (props changed) stable/9/sys/contrib/dev/acpica/ (props changed) stable/9/sys/contrib/octeon-sdk/ (props changed) stable/9/sys/contrib/pf/ (props changed) stable/9/sys/contrib/x86emu/ (props changed) Modified: stable/9/sys/kern/vfs_subr.c ============================================================================== --- stable/9/sys/kern/vfs_subr.c Tue Oct 11 13:19:56 2011 (r226250) +++ stable/9/sys/kern/vfs_subr.c Tue Oct 11 13:22:35 2011 (r226251) @@ -348,6 +348,38 @@ SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_FIRST, /* * Mark a mount point as busy. Used to synchronize access and to delay * unmounting. Eventually, mountlist_mtx is not released on failure. + * + * vfs_busy() is a custom lock, it can block the caller. + * vfs_busy() only sleeps if the unmount is active on the mount point. + * For a mountpoint mp, vfs_busy-enforced lock is before lock of any + * vnode belonging to mp. + * + * Lookup uses vfs_busy() to traverse mount points. + * root fs var fs + * / vnode lock A / vnode lock (/var) D + * /var vnode lock B /log vnode lock(/var/log) E + * vfs_busy lock C vfs_busy lock F + * + * Within each file system, the lock order is C->A->B and F->D->E. + * + * When traversing across mounts, the system follows that lock order: + * + * C->A->B + * | + * +->F->D->E + * + * The lookup() process for namei("/var") illustrates the process: + * VOP_LOOKUP() obtains B while A is held + * vfs_busy() obtains a shared lock on F while A and B are held + * vput() releases lock on B + * vput() releases lock on A + * VFS_ROOT() obtains lock on D while shared lock on F is held + * vfs_unbusy() releases shared lock on F + * vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A. + * Attempt to lock A (instead of vp_crossmp) while D is held would + * violate the global order, causing deadlocks. + * + * dounmount() locks B while F is drained. */ int vfs_busy(struct mount *mp, int flags)
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