From: Miklos Szeredi Subject: Re: [PATCH 2/3] enhanced syscall ESTALE error handling (v2) Date: Sat, 02 Feb 2008 09:00:44 +0100 Message-ID: References: <4790C768.4080207@redhat.com> <47A387D4.70605@redhat.com> <47A39471.4010105@redhat.com> <47A39D8F.9010003@redhat.com> Cc: miklos@szeredi.hu, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, akpm@linux-foundation.org, trond.myklebust@fys.uio.no, linux-fsdevel@vger.kernel.org To: staubach@redhat.com Return-path: Received: from fxip-0047f.externet.hu ([88.209.222.127]:40672 "EHLO pomaz-ex.szeredi.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756304AbYBBIBQ (ORCPT ); Sat, 2 Feb 2008 03:01:16 -0500 In-reply-to: <47A39D8F.9010003@redhat.com> (message from Peter Staubach on Fri, 01 Feb 2008 17:30:39 -0500) Sender: linux-nfs-owner@vger.kernel.org List-ID: > >>> > >> Would you describe the situation that would cause the kernel to > >> go into an infinite loop, please? > >> > > > > The patch basically does: > > > > do { > > ... > > error = inode->i_op->foo() > > ... > > } while (error == ESTALE); > > > > What is the guarantee, that ->foo() will not always return ESTALE? > > You skimmed over some stuff, like the pathname lookup component > contained in the first set of dots... > > I can't guarantee that ->foo() won't always return ESTALE. > > That said, the loop is not unbreakable. At least for NFS, a signal > to the process will interrupt the loop because the error returned > will change from ESTALE to EINTR. In FUSE interrupts are sent to userspace, and the filesystem decides what to do with them. So it is entirely possible and valid for a filesystem to ignore an interrupt. If an operation was non-blocking (such as one returning an error), then there would in fact be no purpose in checking interrupts. So while sending a signal might reliably work in NFS to break out of the loop, it does not necessarily work for other filesystems, and fuse may not be the only one affected. Also up till now, returning ESTALE in a fuse filesystem was a perfectly valid thing to do. This patch changes the behavior of that rather drastically. There might be installed systems that rely on current behavior, and we want to avoid breaking those on a kernel upgrade. A few solutions come to mind, perhaps the best is to introduce a kernel internal errno value (ERETRYSTALE), that forces the relevant system calls to be retried. NFS could transform ESTALE errors to ERETRYSTALE and get the desired behavior, while other filesystems would not be affected. Miklos