illumos gate

zfsctl_snapshot_inactive() leaks a hold on the dvp (directory vnode) if v_count > 1.

reproduce by:

create a fs with 100 snapshots.

have a thread do:
while true; do ls -l /test/snaps/.zfs/snapshot >/dev/null; done

have another thread do:
while true; do zfs promote test/clone; zfs promote test/snaps; done

use dtrace to delay & observe:

dtrace w -xd \\
-n 'vn_rele:entry/args⁰ == (void*)0xffffff01dd42ce80ULL/{[stack()]=count(); chill(100000);}' \\
-n 'zfsctl_snapshot_inactive:entry{ if (args[0]->v_count > 1) trace(args[0]->v_count); self->vp=args[0];}' \\
-n 'gfs_vop_inactive:entry/callers["zfsctl_snapshot_inactive"]/{self->good=1; [stack()]=count()}' \\
-n 'zfsctl_snapshot_inactive:return{if (self>good) self->good=0; else printf("bad return");}' \\
n 'gfs_dir_lookup:return/callers["zfsctl_snapshot_inactive"] && self>vp->v_count > 1/{trace(self->vp->v_count)}'

the address is found by selecting one of the output of this at random:

dtrace -n 'zfsctl_snapshot_inactive:entry{print(args[0]);'

when you see "bad return", we have hit the bug. Then doing "zfs umount test/snaps" will fail with EBUSY.

When we hit this case, we also leak the hold on the target vnode (vn). When the inactive callback is called on a vnode with v_count > 1, it needs to be decremented.