Author: mjg Date: Sun Sep 1 14:01:09 2019 New Revision: 351656 URL: https://svnweb.freebsd.org/changeset/base/351656
Log: vfs: stop refing freed mount points in vop_stdgetwritemount The code used blindly ref based on an unsafely red address and then would backpedal if necessary. This was safe in terms of memory access since mounts are type-stable, but made for a potential a bug where the mount was reused and had the count reset to 0 before this code decreased it. Reviewed by: kib Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D21411 Modified: head/sys/kern/vfs_default.c Modified: head/sys/kern/vfs_default.c ============================================================================== --- head/sys/kern/vfs_default.c Sun Sep 1 10:39:16 2019 (r351655) +++ head/sys/kern/vfs_default.c Sun Sep 1 14:01:09 2019 (r351656) @@ -588,22 +588,28 @@ vop_stdgetwritemount(ap) } */ *ap; { struct mount *mp; + struct vnode *vp; /* - * XXX Since this is called unlocked we may be recycled while - * attempting to ref the mount. If this is the case or mountpoint - * will be set to NULL. We only have to prevent this call from - * returning with a ref to an incorrect mountpoint. It is not - * harmful to return with a ref to our previous mountpoint. + * Note that having a reference does not prevent forced unmount from + * setting ->v_mount to NULL after the lock gets released. This is of + * no consequence for typical consumers (most notably vn_start_write) + * since in this case the vnode is VI_DOOMED. Unmount might have + * progressed far enough that its completion is only delayed by the + * reference obtained here. The consumer only needs to concern itself + * with releasing it. */ - mp = ap->a_vp->v_mount; - if (mp != NULL) { - vfs_ref(mp); - if (mp != ap->a_vp->v_mount) { - vfs_rel(mp); - mp = NULL; - } + vp = ap->a_vp; + mp = vp->v_mount; + MNT_ILOCK(mp); + if (mp != vp->v_mount) { + MNT_IUNLOCK(mp); + mp = NULL; + goto out; } + MNT_REF(mp); + MNT_IUNLOCK(mp); +out: *(ap->a_mpp) = mp; return (0); } _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
