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Date: Wed, 14 Jan 2015 13:06:17 -0500
Message-ID: <CAHQdGtTY6hdXQZGWgKgay6cRDTqruvHoMJTSXhdbDOoBSiWoJA@mail.gmail.com>
Subject: Re: [PATCH] nfs: don't call blocking operations while !TASK_RUNNING
From: Trond Myklebust <trond.myklebust@primarydata.com>
To: Jeff Layton <jeff.layton@primarydata.com>
Cc: Bruce Fields <bfields@fieldses.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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On Wed, Jan 14, 2015 at 12:27 PM, Jeff Layton
<jeff.layton@primarydata.com> wrote:
> On Wed, 14 Jan 2015 12:13:43 -0500
> Trond Myklebust <trond.myklebust@primarydata.com> wrote:
>
>> On Wed, Jan 14, 2015 at 10:32 AM, Jeff Layton <jlayton@primarydata.com> wrote:
>> > Bruce reported seeing this warning pop when mounting using v4.1:
>> >
>> >      ------------[ cut here ]------------
>> >      WARNING: CPU: 1 PID: 1121 at kernel/sched/core.c:7300 __might_sleep+0xbd/0xd0()
>> >     do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffff810ff58f>] prepare_to_wait+0x2f/0x90
>> >     Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver nfs lockd grace sunrpc fscache ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw snd_hda_codec_generic snd_hda_intel snd_hda_controller snd_hda_codec snd_hwdep snd_pcm snd_timer ppdev joydev snd virtio_console virtio_balloon pcspkr serio_raw parport_pc parport pvpanic floppy soundcore i2c_piix4 virtio_blk virtio_net qxl drm_kms_helper ttm drm virtio_pci virtio_ring ata_generic virtio pata_acpi
>> >     CPU: 1 PID: 1121 Comm: nfsv4.1-svc Not tainted 3.19.0-rc4+ #25
>> >     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140709_153950- 04/01/2014
>> >      0000000000000000 000000004e5e3f73 ffff8800b998fb48 ffffffff8186ac78
>> >      0000000000000000 ffff8800b998fba0 ffff8800b998fb88 ffffffff810ac9da
>> >      ffff8800b998fb68 ffffffff81c923e7 00000000000004d9 0000000000000000
>> >     Call Trace:
>> >      [<ffffffff8186ac78>] dump_stack+0x4c/0x65
>> >      [<ffffffff810ac9da>] warn_slowpath_common+0x8a/0xc0
>> >      [<ffffffff810aca65>] warn_slowpath_fmt+0x55/0x70
>> >      [<ffffffff810ff58f>] ? prepare_to_wait+0x2f/0x90
>> >      [<ffffffff810ff58f>] ? prepare_to_wait+0x2f/0x90
>> >      [<ffffffff810dd2ad>] __might_sleep+0xbd/0xd0
>> >      [<ffffffff8124c973>] kmem_cache_alloc_trace+0x243/0x430
>> >      [<ffffffff810d941e>] ? groups_alloc+0x3e/0x130
>> >      [<ffffffff810d941e>] groups_alloc+0x3e/0x130
>> >      [<ffffffffa0301b1e>] svcauth_unix_accept+0x16e/0x290 [sunrpc]
>> >      [<ffffffffa0300571>] svc_authenticate+0xe1/0xf0 [sunrpc]
>> >      [<ffffffffa02fc564>] svc_process_common+0x244/0x6a0 [sunrpc]
>> >      [<ffffffffa02fd044>] bc_svc_process+0x1c4/0x260 [sunrpc]
>> >      [<ffffffffa03d5478>] nfs41_callback_svc+0x128/0x1f0 [nfsv4]
>> >      [<ffffffff810ff970>] ? wait_woken+0xc0/0xc0
>> >      [<ffffffffa03d5350>] ? nfs4_callback_svc+0x60/0x60 [nfsv4]
>> >      [<ffffffff810d45bf>] kthread+0x11f/0x140
>> >      [<ffffffff810ea815>] ? local_clock+0x15/0x30
>> >      [<ffffffff810d44a0>] ? kthread_create_on_node+0x250/0x250
>> >      [<ffffffff81874bfc>] ret_from_fork+0x7c/0xb0
>> >      [<ffffffff810d44a0>] ? kthread_create_on_node+0x250/0x250
>> >     ---[ end trace 675220a11e30f4f2 ]---
>> >
>> > nfs41_callback_svc does most of its work while in TASK_INTERRUPTIBLE,
>> > which is just wrong. Fix that by finishing the wait immediately if we've
>> > found that the list has something on it.
>>
>> ACK.
>>
>> > Also, we don't expect this kthread to accept signals, so we should be
>> > using a TASK_UNINTERRUPTIBLE sleep instead.
>>
>> Umm... Won't that end up triggering the hung task watchdog for every
>> 120seconds with no callback activity?
>>
>
> Doh! You're correct.
>
> What's the right way to do this then? Do we need to use
> schedule_timeout and wake up every 100s or so? Using TASK_INTERRUPTIBLE
> to work around the watchdog seems wrong but I guess we can live with
> that in the short term if it's the only way.

The alternative is to use TASK_KILLABLE. That's a little more
restrictive, but still a PITA. Note the both TASK_INTERRUPTIBLE and
TASK_KILLABLE will need something to handle that signalled() case,
otherwise we end up with a permanent busy-waiting loop.

-- 
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@primarydata.com