Date: Wed, 05 Mar 2014 11:41:24 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org>, Bryan Drewery <bdrewery@FreeBSD.org>, Konstantin Belousov <kib@FreeBSD.org>, Alan Cox <alc@FreeBSD.org> Cc: freebsd-current@FreeBSD.org Subject: Re: panic: lockmgr still held [tmpfs] [vm_map_remove()->vdropl()] (r262186: Thu Feb 20) Message-ID: <5316F144.1000105@FreeBSD.org> In-Reply-To: <201403041145.48425.jhb@freebsd.org> References: <53109ACB.20102@FreeBSD.org> <201403031306.59405.jhb@freebsd.org> <53153F43.6010506@FreeBSD.org> <201403041145.48425.jhb@freebsd.org>
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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. -- Andriy Gapon
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