From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:40476 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934403AbdEVWE1 (ORCPT ); Mon, 22 May 2017 18:04:27 -0400 Date: Mon, 22 May 2017 18:04:24 -0400 From: Brian Foster Subject: Re: [PATCH] xfs: use atomic to provide buffer I/O accounting serialization Message-ID: <20170522220424.GA4456@bfoster.bfoster> References: <1495477751-3742-1-git-send-email-bfoster@redhat.com> <20170522190510.GA17100@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170522190510.GA17100@infradead.org> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Christoph Hellwig Cc: linux-xfs@vger.kernel.org On Mon, May 22, 2017 at 12:05:10PM -0700, Christoph Hellwig wrote: > On Mon, May 22, 2017 at 02:29:11PM -0400, Brian Foster wrote: > > We've had user reports of unmount hangs in xfs_wait_buftarg() that > > analysis shows is due to btp->bt_io_count == -1. bt_io_count > > represents the count of in-flight asynchronous buffers and thus > > should always be >= 0. xfs_wait_buftarg() waits for this value to > > stabilize to zero in order to ensure that all untracked (with > > respect to the lru) buffers have completed I/O processing before > > unmount proceeds to tear down in-core data structures. > > > > The value of -1 implies an I/O accounting decrement race. Indeed, > > the fact that xfs_buf_ioacct_dec() is called from xfs_buf_rele() > > (where the buffer lock is no longer held) means that bp->b_flags can > > be updated from an unsafe context. While a user-level reproducer is > > currently not available, some intrusive hacks to run racing buffer > > lookups/ioacct/releases from multiple threads was used to > > successfully manufacture this problem. > > > > Existing callers do not expect to acquire the buffer lock from > > xfs_buf_rele(). Therefore, we can not safely update ->b_flags from > > this context. To close the race, replace the in-flight buffer flag > > with a per-buffer atomic for tracking accounting against the > > buftarg. This field resides in a hole in the existing data structure > > and thus does not increase the size of xfs_buf. > > I hate these uses of atomic_t as binary flags. Can you use > test_and_set_bit and friends wit a bitop? This would require > an unsigned long which an actually be larger than an atomic_t, > but it's both cleaner and provides headroom for additional atomic flags > in the future. I thought it may be a little confusing to have multiple sets of flags for a buffer, hence the counter (even though it is logically a flag). But I'm fine with it for now if we don't mind wasting the extra space. Though I suppose we could also add a smaller field and use cmpxchg() to set and clear it... thoughts? Brian