On Apr 7, 2014, at 18:35, Jan Kara <jack@xxxxxxx> wrote: > On Mon 07-04-14 18:02:16, Trond Myklebust wrote: >> On Mon, 2014-04-07 at 22:27 +0200, Jan Kara wrote: >>> On Mon 07-04-14 10:10:27, Trond Myklebust wrote: >>>> On Apr 6, 2014, at 23:50, NeilBrown <neilb@xxxxxxx> wrote: >>>>> I've just hit a deadlock in NFS that seems very strange. >>>>> The kernel is 3.14-rc8 which some local changes which shouldn't affect the >>>>> deadlocking code. >>>>> >>>>> Shortly after umounting the NFS filesystem with "umount -f" (though I don't >>>>> think the -f is important), I ran "sync". >>>>> >>>>> The sync is now stuck in >>>>> >>>>> [<ffffffff81197fc1>] sync_inodes_sb+0xa1/0x1c0 >>>>> [<ffffffff8119cd99>] sync_inodes_one_sb+0x19/0x20 >>>>> [<ffffffff81173372>] iterate_supers+0xb2/0x110 >>>>> [<ffffffff8119cfd0>] sys_sync+0x30/0x90 >>>>> [<ffffffff81aa4622>] system_call_fastpath+0x16/0x1b >>>>> [<ffffffffffffffff>] 0xffffffffffffffff >>>>> >>>>> while kworker/u16:1 is stuck: >>>>> >>>>> [<ffffffff815420b3>] call_rwsem_down_write_failed+0x13/0x20 >>>>> [<ffffffff81172889>] deactivate_super+0x39/0x60 >>>>> [<ffffffff812d56f1>] nfs_sb_deactive+0x21/0x30 >>>>> [<ffffffff812d2ef9>] __put_nfs_open_context+0xc9/0x100 >>>>> [<ffffffff812d2f3b>] put_nfs_open_context+0xb/0x10 >>>>> [<ffffffff812ddd14>] nfs_commitdata_release+0x14/0x30 >>>>> [<ffffffff812ddd4a>] nfs_commit_release+0x1a/0x20 >>>>> [<ffffffff81a45a05>] rpc_free_task+0x25/0x70 >>>>> [<ffffffff81a45fd8>] rpc_do_put_task+0x78/0x80 >>>>> [<ffffffff81a45feb>] rpc_put_task+0xb/0x10 >>>>> [<ffffffff812de3fe>] nfs_initiate_commit+0xce/0x110 >>>>> [<ffffffff812df112>] nfs_commit_list+0x62/0x90 >>>>> [<ffffffff812dfd26>] nfs_commit_inode+0xa6/0x170 >>>>> [<ffffffff812dfe4d>] nfs_write_inode+0x5d/0xa0 >>>>> [<ffffffff81300d69>] nfs4_write_inode+0x9/0x10 >>>>> [<ffffffff811978ec>] __writeback_single_inode+0x10c/0x2c0 >>>>> [<ffffffff811987ea>] writeback_sb_inodes+0x2ca/0x450 >>>>> [<ffffffff81198b2c>] wb_writeback+0xec/0x320 >>>>> [<ffffffff81199365>] bdi_writeback_workfn+0x115/0x4c0 >>>>> [<ffffffff810a595b>] process_one_work+0x16b/0x430 >>>>> [<ffffffff810a6619>] worker_thread+0x119/0x3a0 >>>>> [<ffffffff810ac2bd>] kthread+0xcd/0xf0 >>>>> [<ffffffff81aa457c>] ret_from_fork+0x7c/0xb0 >>>>> [<ffffffffffffffff>] 0xffffffffffffffff >>>>> >>>>> >>>>> So sync is holding sb->s_umount, queued some bdi work on the filesystem >>>>> and is waiting for it to complete. Mean while, that work has (I think) >>>>> submitted a 'commit' (via ->write_inode) and that commit wants to >>>>> deactivate_super and so needs to get ->s_umount. >>>>> >>>>> I suspect this could happen even more easily with a lazy unmount. >>>>> >>>>> It seems that this commit request is that last thing that is keeping >>>>> ->s_active elevated and it deadlocks trying to drop the last s_active. >>>>> >>>>> I have no idea how to fix it.... help? >>>>> >>>> >>>> The problem seems to be the use of iterate_supers(), which grabs a >>>> passive reference, and conflicts with our use of an active reference in >>>> the open context. >>> Yeah, we cannot really do otherwise in iterate_supers() - we have to grab >>> some superblock reference and we don't really want to get an active one >>> since that would result in spurious EBUSY returns from umount. >>> >>> Cannot we just punt the deactivate_super() call to a workqueue to avoid >>> this deadlock? It's a bit ugly but it should do the trick. Or is >>> nfs_sb_deactive() called too often and we'd see some adverse effects for >>> that? We could also offload it to workqueue only in the special case where >>> sb->s_active == 1. That should be really rare then but it's a bit ugly >>> poking in VFS internals. >> >> The activate/deactivate super is basically there to save our bacon when >> NFS file state extends beyond the usual VFS path walk, open() and >> close(). Examples include sillyrename and NFSv4 delegations. Even >> ordinary read and write state can extend beyond close() if the user >> decides to 'kill -9' in the wrong places. >> In most of these situations, we need to keep a dentry around until we're >> finished, which means that we want to keep the super block alive too. > Yeah, that makes sense. But offloading dropping of sb reference to a > workqueue would work then, wouldn't it? Could we perhaps have a helper in the VFS that can optimise away the case where s->s_active > 1? _________________________________ Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
