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

[Konstantin Belousov]

On Wed, Mar 05, 2014 at 02:21:04PM -0500, John Baldwin wrote:
> On Wednesday, March 05, 2014 6:07:23 am Konstantin Belousov wrote:
> > 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.
> 
> Ok, my only worry is in the case of Bryan's panic, the hold count on the vnode
> was already zero before vhold() was called, so is it possible that it is a stale
> pointer or is there some other implicit reference that prevents that?  If it can't
> be stale, I think deferring the vdrop() is fine.

The object->un_pager.swp.swp_tmpfs is cleared under the object lock
before the vnode is reclaimed, i.e. long before the vnode can be freed.
swp_tmpfs should be kept in sync with the OBJ_TMPFS flag, so the
vhold() is safe while flag is set and object is locked.
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