From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com ([141.146.126.69]:35513 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751145AbaHMPVd (ORCPT ); Wed, 13 Aug 2014 11:21:33 -0400 Date: Wed, 13 Aug 2014 23:20:46 +0800 From: Liu Bo To: Martin Steigerwald Cc: linux-btrfs , Chris Mason , miaox@cn.fujitsu.com, Marc MERLIN , =?iso-8859-1?Q?Torbj=F8rn?= Subject: Re: [PATCH] Btrfs: fix task hang under heavy compressed write Message-ID: <20140813152045.GA9273@localhost.localdomain> Reply-To: bo.li.liu@oracle.com References: <1407829499-21902-1-git-send-email-bo.li.liu@oracle.com> <2364156.aMAqnATvIX@merkaba> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <2364156.aMAqnATvIX@merkaba> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Wed, Aug 13, 2014 at 01:54:40PM +0200, Martin Steigerwald wrote: > Am Dienstag, 12. August 2014, 15:44:59 schrieb Liu Bo: > > This has been reported and discussed for a long time, and this hang occurs > > in both 3.15 and 3.16. > > Liu, is this safe for testing yet? Yes, I've confirmed that this hang doesn't occur by running my tests for 2 days(usually it hangs in 2 hours). But... As Chris said in the thread, this is more a workaround, there're other potential issues that would lead to similar deadlock. I'm trying to write a real fix instead of a workaround. thanks, -liubo > > Thanks, > Martin > > > Btrfs now migrates to use kernel workqueue, but it introduces this hang > > problem. > > > > Btrfs has a kind of work queued as an ordered way, which means that its > > ordered_func() must be processed in the way of FIFO, so it usually looks > > like -- > > > > normal_work_helper(arg) > > work = container_of(arg, struct btrfs_work, normal_work); > > > > work->func() <---- (we name it work X) > > for ordered_work in wq->ordered_list > > ordered_work->ordered_func() > > ordered_work->ordered_free() > > > > The hang is a rare case, first when we find free space, we get an uncached > > block group, then we go to read its free space cache inode for free space > > information, so it will > > > > file a readahead request > > btrfs_readpages() > > for page that is not in page cache > > __do_readpage() > > submit_extent_page() > > btrfs_submit_bio_hook() > > btrfs_bio_wq_end_io() > > submit_bio() > > end_workqueue_bio() <--(ret by the 1st > > endio) queue a work(named work Y) for the 2nd also the real endio() > > > > So the hang occurs when work Y's work_struct and work X's work_struct > > happens to share the same address. > > > > A bit more explanation, > > > > A,B,C -- struct btrfs_work > > arg -- struct work_struct > > > > kthread: > > worker_thread() > > pick up a work_struct from @worklist > > process_one_work(arg) > > worker->current_work = arg; <-- arg is A->normal_work > > worker->current_func(arg) > > normal_work_helper(arg) > > A = container_of(arg, struct btrfs_work, normal_work); > > > > A->func() > > A->ordered_func() > > A->ordered_free() <-- A gets freed > > > > B->ordered_func() > > submit_compressed_extents() > > find_free_extent() > > load_free_space_inode() > > ... <-- (the above readhead stack) > > end_workqueue_bio() > > btrfs_queue_work(work C) > > B->ordered_free() > > > > As if work A has a high priority in wq->ordered_list and there are more > > ordered works queued after it, such as B->ordered_func(), its memory could > > have been freed before normal_work_helper() returns, which means that > > kernel workqueue code worker_thread() still has worker->current_work > > pointer to be work A->normal_work's, ie. arg's address. > > > > Meanwhile, work C is allocated after work A is freed, work C->normal_work > > and work A->normal_work are likely to share the same address(I confirmed > > this with ftrace output, so I'm not just guessing, it's rare though). > > > > When another kthread picks up work C->normal_work to process, and finds our > > kthread is processing it(see find_worker_executing_work()), it'll think > > work C as a collision and skip then, which ends up nobody processing work C. > > > > So the situation is that our kthread is waiting forever on work C. > > > > The key point is that they shouldn't have the same address, so this defers > > ->ordered_free() and does a batched free to avoid that. > > > > Signed-off-by: Liu Bo > > --- > > fs/btrfs/async-thread.c | 12 ++++++++++-- > > 1 file changed, 10 insertions(+), 2 deletions(-) > > > > diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c > > index 5a201d8..2ac01b3 100644 > > --- a/fs/btrfs/async-thread.c > > +++ b/fs/btrfs/async-thread.c > > @@ -195,6 +195,7 @@ static void run_ordered_work(struct __btrfs_workqueue > > *wq) struct btrfs_work *work; > > spinlock_t *lock = &wq->list_lock; > > unsigned long flags; > > + LIST_HEAD(free_list); > > > > while (1) { > > spin_lock_irqsave(lock, flags); > > @@ -219,17 +220,24 @@ static void run_ordered_work(struct __btrfs_workqueue > > *wq) > > > > /* now take the lock again and drop our item from the list */ > > spin_lock_irqsave(lock, flags); > > - list_del(&work->ordered_list); > > + list_move_tail(&work->ordered_list, &free_list); > > spin_unlock_irqrestore(lock, flags); > > > > /* > > * we don't want to call the ordered free functions > > * with the lock held though > > */ > > + } > > + spin_unlock_irqrestore(lock, flags); > > + > > + while (!list_empty(&free_list)) { > > + work = list_entry(free_list.next, struct btrfs_work, > > + ordered_list); > > + > > + list_del(&work->ordered_list); > > work->ordered_free(work); > > trace_btrfs_all_work_done(work); > > } > > - spin_unlock_irqrestore(lock, flags); > > } > > > > static void normal_work_helper(struct work_struct *arg) > > -- > Martin 'Helios' Steigerwald - http://www.Lichtvoll.de > GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7