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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