Date: Sun, 13 Nov 2016 21:49:51 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r308618 - in head/sys: kern ufs/ffs Message-ID: <201611132149.uADLnpac016426@repo.freebsd.org>
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Author: kib Date: Sun Nov 13 21:49:51 2016 New Revision: 308618 URL: https://svnweb.freebsd.org/changeset/base/308618 Log: Provide simple mutual exclusion between mount point update and unmount. Currently mount update keeps vfs_busy(9) reference on the mount point during MNT_UPDATE VFS_MOUNT() vfsops call. This already provides the exclusion, but is problematic for filesystems which need to perform namei(9) during VFS_MOUNT(MNT_UPDATE) operations, e.g. to refresh mnt_from path, because namei(9) must not be called while the vfs_busy(9) reference is owned. Check for MNT_UPDATE flag before setting MNTK_UNMOUNT, and for MNTK_UNMOUNT before entering innards of vfs_domount_update(), failing syscalls with EBUSY if conflict is detected. Keep vfs_busy(9) reference around VFS_MOUNT(MNT_UPDATE) calls still to not change VFS KPI. In the update path in ffs_mount(), drop vfs_busy() reference around namei(), which is now safe due to unmount never executing in parallel with VFS_MOUNT(MNT_UPDATE), and which avoids the deadlock. Reported and tested by: pho Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Modified: head/sys/kern/vfs_mount.c head/sys/ufs/ffs/ffs_vfsops.c Modified: head/sys/kern/vfs_mount.c ============================================================================== --- head/sys/kern/vfs_mount.c Sun Nov 13 21:39:55 2016 (r308617) +++ head/sys/kern/vfs_mount.c Sun Nov 13 21:49:51 2016 (r308618) @@ -940,6 +940,11 @@ vfs_domount_update( VOP_UNLOCK(vp, 0); MNT_ILOCK(mp); + if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) { + MNT_IUNLOCK(mp); + error = EBUSY; + goto end; + } mp->mnt_flag &= ~MNT_UPDATEMASK; mp->mnt_flag |= fsflags & (MNT_RELOAD | MNT_FORCE | MNT_UPDATE | MNT_SNAPSHOT | MNT_ROOTFS | MNT_UPDATEMASK | MNT_RDONLY); @@ -1299,6 +1304,7 @@ dounmount(struct mount *mp, int flags, s vn_start_write(NULL, &mp, V_WAIT | V_MNTREF); MNT_ILOCK(mp); if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 || + (mp->mnt_flag & MNT_UPDATE) != 0 || !TAILQ_EMPTY(&mp->mnt_uppers)) { dounmount_cleanup(mp, coveredvp, 0); return (EBUSY); Modified: head/sys/ufs/ffs/ffs_vfsops.c ============================================================================== --- head/sys/ufs/ffs/ffs_vfsops.c Sun Nov 13 21:39:55 2016 (r308617) +++ head/sys/ufs/ffs/ffs_vfsops.c Sun Nov 13 21:49:51 2016 (r308618) @@ -147,7 +147,7 @@ ffs_mount(struct mount *mp) struct ufsmount *ump = NULL; struct fs *fs; pid_t fsckpid = 0; - int error, flags; + int error, error1, flags; uint64_t mntorflags; accmode_t accmode; struct nameidata ndp; @@ -453,6 +453,11 @@ ffs_mount(struct mount *mp) */ if (mp->mnt_flag & MNT_SNAPSHOT) return (ffs_snapshot(mp, fspec)); + + /* + * Must not call namei() while owning busy ref. + */ + vfs_unbusy(mp); } /* @@ -460,7 +465,18 @@ ffs_mount(struct mount *mp) * and verify that it refers to a sensible disk device. */ NDINIT(&ndp, LOOKUP, FOLLOW | LOCKLEAF, UIO_SYSSPACE, fspec, td); - if ((error = namei(&ndp)) != 0) + error = namei(&ndp); + if ((mp->mnt_flag & MNT_UPDATE) != 0) { + /* + * Unmount does not start if MNT_UPDATE is set. Mount + * update busies mp before setting MNT_UPDATE. We + * must be able to retain our busy ref succesfully, + * without sleep. + */ + error1 = vfs_busy(mp, MBF_NOWAIT); + MPASS(error1 == 0); + } + if (error != 0) return (error); NDFREE(&ndp, NDF_ONLY_PNBUF); devvp = ndp.ni_vp;
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