Re: panic: lockmgr still held [tmpfs] [vm_map_remove()->vdropl()] (r262186: Thu Feb 20)

On Wed, Mar 05, 2014 at 11:41:24AM +0200, Andriy Gapon wrote:
> on 04/03/2014 18:45 John Baldwin said the following:
> > So I'm not sure how to fix this.  The crash is in this code in 
> > vm_object_deallocate():
> > 
> > 			if (object->type == OBJT_SWAP &&
> > 			    (object->flags & OBJ_TMPFS) != 0) {
> > 				vp = object->un_pager.swp.swp_tmpfs;
> > 				vhold(vp);
> > 				VM_OBJECT_WUNLOCK(object);
> > 				vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
> > 				vdrop(vp);
> > 				VM_OBJECT_WLOCK(object);
> > 				if (object->type == OBJT_DEAD ||
> > 				    object->ref_count != 1) {
> > 					VM_OBJECT_WUNLOCK(object);
> > 					VOP_UNLOCK(vp, 0);
> > 					return;
> > 				}
> > 				if ((object->flags & OBJ_TMPFS) != 0)
> > 					VOP_UNSET_TEXT(vp);
> > 				VOP_UNLOCK(vp, 0);
> > 			}
> > 
> > The vdrop() is dropping the count to zero and trying to free the vnode.  The 
> > real problem I think is that swp_tmpfs doesn't have an implicit vhold() on the 
> > vnode, so in this case, the code is doing a vhold/vn_lock/vdrop of an already-
> > free vnode.  For OBJT_VNODE objects, the reference from the object back to the 
> > vnode holds a vref() that gets released by a vput() in 
> > vm_object_vndeallocate().
> > 
> > One fix might be to chagne smp_tmpfs to hold a vhold reference.  This is 
> > untested but might work (but I'm also not sure that this is the right thing in 
> > that I don't know what other effects it might have).
> 
> I agree with your analysis, but I don't think that a filesystem holding its own
> vnode is a good idea.  If I am not mistaken, that would prevent tmpfs vnodes
> from going to free list.
> I'd rather try to modify vm_object_deallocate() code.  E.g. vdrop() could be
> called after VOP_UNLOCK().  Alternatively, the code could handle a doomed vnode
> in a different way.

I agree with Andrey, it is just a bug to vdrop() before unlock.
Please try this.

diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
index 8683e2f..787b18b 100644
--- a/sys/vm/vm_object.c
+++ b/sys/vm/vm_object.c
_at__at_ -536,17 +536,18 _at__at_ vm_object_deallocate(vm_object_t object)
 				vhold(vp);
 				VM_OBJECT_WUNLOCK(object);
 				vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
-				vdrop(vp);
 				VM_OBJECT_WLOCK(object);
 				if (object->type == OBJT_DEAD ||
 				    object->ref_count != 1) {
 					VM_OBJECT_WUNLOCK(object);
 					VOP_UNLOCK(vp, 0);
+					vdrop(vp);
 					return;
 				}
 				if ((object->flags & OBJ_TMPFS) != 0)
 					VOP_UNSET_TEXT(vp);
 				VOP_UNLOCK(vp, 0);
+				vdrop(vp);
 			}
 			if (object->shadow_count == 0 &&
 			    object->handle == NULL &&