K 10
svn:author
V 3
mjg
K 8
svn:date
V 27
2020-02-10T22:28:54.808634Z
K 7
svn:log
V 712
vfs: fix device count leak on vrele racing with vgone

The race is:

CPU1                                CPU2
                                    devfs_reclaim_vchr
make v_usecount 0
                                      VI_LOCK
                                      sees v_usecount == 0, no updates
                                      vp->v_rdev = NULL;
                                      ...
                                      VI_UNLOCK
VI_LOCK
v_decr_devcount
  sees v_rdev == NULL, no updates

In this scenario si_devcount decrement is not performed.

Note this can only happen if the vnode lock is not held.

Reviewed by:	kib
Tested by:	pho
Differential Revision:	https://reviews.freebsd.org/D23529

END