Re: swap: nfs: Sleeping function called from an rcu read section in nfs_swap_activate

From: Jeff Layton
Date: Thu May 28 2015 - 07:24:56 EST

On Thu, 28 May 2015 09:26:19 +0100
Mel Gorman <mgorman@xxxxxxx> wrote:

> On Tue, May 26, 2015 at 09:29:29PM -0400, Jeff Layton wrote:
> > On Tue, 26 May 2015 09:56:14 -0400
> > Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> wrote:
> >
> > > On Tue, 26 May 2015 15:20:46 +0200
> > > Jerome Marchand <jmarchan@xxxxxxxxxx> wrote:
> > >
> > > >
> > > > Commit dad2b015 added an rcu read lock around the call to xs_swapper()
> > > > in nfs_activate()/deactivate(), which can sleep, thus raising a bug at
> > > > each swapon and swapoff over NFS.
> > > > I'm not sure if this is related or not, but swapoff also triggers the
> > > > WARN_ON(sk->sk_forward_alloc) in sk_clear_memalloc().
> > > >
> > > > [ 243.668067] ===============================
> > > > [ 243.668665] [ INFO: suspicious RCU usage. ]
> > > > [ 243.669293] 4.1.0-rc1-lock_stat-dbg-next-20150430+ #235 Not tainted
> > > > [ 243.670301] -------------------------------
> > > > [ 243.670905] include/linux/rcupdate.h:570 Illegal context switch in RCU read-side critical section!
> > > > [ 243.672163]
> > > > other info that might help us debug this:
> > > >
> > > > [ 243.673025]
> > > > rcu_scheduler_active = 1, debug_locks = 0
> > > > [ 243.673565] 2 locks held by swapon/1176:
> > > > [ 243.673893] #0: (&sb->s_type->i_mutex_key#17){+.+.+.}, at: [<ffffffff812385e0>] SyS_swapon+0x2b0/0x1000
> > > > [ 243.674758] #1: (rcu_read_lock){......}, at: [<ffffffffa036fd75>] nfs_swap_activate+0x5/0x180 [nfs]
> > > > [ 243.675591]
> > > > stack backtrace:
> > > > [ 243.675957] CPU: 0 PID: 1176 Comm: swapon Not tainted 4.1.0-rc1-lock_stat-dbg-next-20150430+ #235
> > > > [ 243.676687] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > > > [ 243.677179] 0000000000000000 00000000ef88d841 ffff88003327bcd8 ffffffff818861f0
> > > > [ 243.677854] 0000000000000000 ffff880078e38000 ffff88003327bd08 ffffffff8110d237
> > > > [ 243.678514] 0000000000000000 ffffffff81c650e4 0000000000000268 ffff880078e38000
> > > > [ 243.679171] Call Trace:
> > > > [ 243.679383] [<ffffffff818861f0>] dump_stack+0x4c/0x65
> > > > [ 243.679811] [<ffffffff8110d237>] lockdep_rcu_suspicious+0xe7/0x120
> > > > [ 243.680348] [<ffffffff810df1bf>] ___might_sleep+0xaf/0x250
> > > > [ 243.680815] [<ffffffff810df3ad>] __might_sleep+0x4d/0x90
> > > > [ 243.681279] [<ffffffff8188bc17>] mutex_lock_nested+0x47/0x430
> > > > [ 243.681762] [<ffffffff811e409c>] static_key_slow_inc+0x7c/0xc0
> > > > [ 243.682264] [<ffffffff8171afa7>] sk_set_memalloc+0x27/0x30
> > > > [ 243.682736] [<ffffffffa012f824>] xs_swapper+0x54/0x60 [sunrpc]
> > > > [ 243.683238] [<ffffffffa036fe03>] nfs_swap_activate+0x93/0x180 [nfs]
> > > > [ 243.683760] [<ffffffffa036fd75>] ? nfs_swap_activate+0x5/0x180 [nfs]
> > > > [ 243.684316] [<ffffffff81238e04>] SyS_swapon+0xad4/0x1000
> > > > [ 243.684766] [<ffffffff818911b0>] ? syscall_return+0x16/0x59
> > > > [ 243.685245] [<ffffffff81890f6e>] system_call_fastpath+0x12/0x76
> > > > [ 243.685743] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:616
> > > > [ 243.686439] in_atomic(): 1, irqs_disabled(): 0, pid: 1176, name: swapon
> > > > [ 243.687053] INFO: lockdep is turned off.
> > > > [ 243.687429] CPU: 0 PID: 1176 Comm: swapon Not tainted 4.1.0-rc1-lock_stat-dbg-next-20150430+ #235
> > > > [ 243.688313] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > > > [ 243.688845] 0000000000000000 00000000ef88d841 ffff88003327bd08 ffffffff818861f0
> > > > [ 243.689570] 0000000000000000 ffff880078e38000 ffff88003327bd38 ffffffff810df29c
> > > > [ 243.690353] ffff880000000001 ffffffff81c650e4 0000000000000268 0000000000000000
> > > > [ 243.691057] Call Trace:
> > > > [ 243.691315] [<ffffffff818861f0>] dump_stack+0x4c/0x65
> > > > [ 243.691785] [<ffffffff810df29c>] ___might_sleep+0x18c/0x250
> > > > [ 243.692306] [<ffffffff810df3ad>] __might_sleep+0x4d/0x90
> > > > [ 243.692807] [<ffffffff8188bc17>] mutex_lock_nested+0x47/0x430
> > > > [ 243.693346] [<ffffffff811e409c>] static_key_slow_inc+0x7c/0xc0
> > > > [ 243.693887] [<ffffffff8171afa7>] sk_set_memalloc+0x27/0x30
> > > > [ 243.694416] [<ffffffffa012f824>] xs_swapper+0x54/0x60 [sunrpc]
> > > > [ 243.694959] [<ffffffffa036fe03>] nfs_swap_activate+0x93/0x180 [nfs]
> > > > [ 243.695535] [<ffffffffa036fd75>] ? nfs_swap_activate+0x5/0x180 [nfs]
> > > > [ 243.696193] [<ffffffff81238e04>] SyS_swapon+0xad4/0x1000
> > > > [ 243.696699] [<ffffffff818911b0>] ? syscall_return+0x16/0x59
> > > > [ 243.697299] [<ffffffff81890f6e>] system_call_fastpath+0x12/0x76
> > > > [ 243.702101] Adding 524284k swap on /mnt/swapfile512. Priority:-2 extents:1 across:524284k FS
> > > > [ 325.151350] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:616
> > > > [ 325.152688] in_atomic(): 1, irqs_disabled(): 0, pid: 1199, name: swapoff
> > > > [ 325.153737] INFO: lockdep is turned off.
> > > > [ 325.154457] CPU: 1 PID: 1199 Comm: swapoff Not tainted 4.1.0-rc1-lock_stat-dbg-next-20150430+ #235
> > > > [ 325.156204] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > > > [ 325.157120] 0000000000000000 00000000a7682b83 ffff88007ac3fce8 ffffffff818861f0
> > > > [ 325.158361] 0000000000000000 ffff880032434c00 ffff88007ac3fd18 ffffffff810df29c
> > > > [ 325.159592] 0000000000000000 ffffffff81c650e4 0000000000000268 0000000000000000
> > > > [ 325.160798] Call Trace:
> > > > [ 325.161251] [<ffffffff818861f0>] dump_stack+0x4c/0x65
> > > > [ 325.162071] [<ffffffff810df29c>] ___might_sleep+0x18c/0x250
> > > > [ 325.163073] [<ffffffff810df3ad>] __might_sleep+0x4d/0x90
> > > > [ 325.163934] [<ffffffff8188bc17>] mutex_lock_nested+0x47/0x430
> > > > [ 325.164927] [<ffffffff8110a00f>] atomic_dec_and_mutex_lock+0x4f/0x70
> > > > [ 325.166020] [<ffffffff811e4107>] __static_key_slow_dec+0x27/0xc0
> > > > [ 325.166942] [<ffffffff811e41c6>] static_key_slow_dec+0x26/0x50
> > > > [ 325.167955] [<ffffffff8171db3f>] sk_clear_memalloc+0x2f/0x80
> > > > [ 325.169075] [<ffffffffa012f811>] xs_swapper+0x41/0x60 [sunrpc]
> > > > [ 325.170241] [<ffffffffa0370447>] nfs_swap_deactivate+0x87/0x170 [nfs]
> > > > [ 325.171276] [<ffffffffa03703c5>] ? nfs_swap_deactivate+0x5/0x170 [nfs]
> > > > [ 325.172349] [<ffffffff81237547>] destroy_swap_extents+0x77/0x90
> > > > [ 325.173754] [<ffffffff8123b225>] SyS_swapoff+0x215/0x600
> > > > [ 325.174726] [<ffffffff81434deb>] ? trace_hardirqs_on_thunk+0x17/0x19
> > > > [ 325.175971] [<ffffffff81890f6e>] system_call_fastpath+0x12/0x76
> > > > [ 325.178052] ------------[ cut here ]------------
> > > > [ 325.178892] WARNING: CPU: 1 PID: 1199 at net/core/sock.c:364 sk_clear_memalloc+0x51/0x80()
> > > > [ 325.180363] Modules linked in: rpcsec_gss_krb5 nfsv4 dns_resolver nfs 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 iosf_mbi crct10dif_pclmul crc32_pclmul crc32c_intel ppdev ghash_clmulni_intel joydev nfsd parport_pc pcspkr virtio_console serio_raw virtio_balloon parport pvpanic i2c_piix4 acpi_cpufreq auth_rpcgss nfs_acl lockd grace sunrpc virtio_blk qxl virtio_net drm_kms_helper ttm drm virtio_pci virtio_ring virtio ata_generic pata_acpi floppy
> > > > [ 325.192279] CPU: 1 PID: 1199 Comm: swapoff Not tainted 4.1.0-rc1-lock_stat-dbg-next-20150430+ #235
> > > > [ 325.193605] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > > > [ 325.194491] 0000000000000000 00000000a7682b83 ffff88007ac3fdf8 ffffffff818861f0
> > > > [ 325.195692] 0000000000000000 0000000000000000 ffff88007ac3fe38 ffffffff810af5ca
> > > > [ 325.196891] ffff88007ac3fe78 ffff88007b068000 ffff88007b484a00 ffff88007b484aa8
> > > > [ 325.198119] Call Trace:
> > > > [ 325.198555] [<ffffffff818861f0>] dump_stack+0x4c/0x65
> > > > [ 325.199380] [<ffffffff810af5ca>] warn_slowpath_common+0x8a/0xc0
> > > > [ 325.200601] [<ffffffff810af6fa>] warn_slowpath_null+0x1a/0x20
> > > > [ 325.201536] [<ffffffff8171db61>] sk_clear_memalloc+0x51/0x80
> > > > [ 325.202468] [<ffffffffa012f811>] xs_swapper+0x41/0x60 [sunrpc]
> > > > [ 325.203398] [<ffffffffa0370447>] nfs_swap_deactivate+0x87/0x170 [nfs]
> > > > [ 325.204426] [<ffffffffa03703c5>] ? nfs_swap_deactivate+0x5/0x170 [nfs]
> > > > [ 325.205456] [<ffffffff81237547>] destroy_swap_extents+0x77/0x90
> > > > [ 325.206406] [<ffffffff8123b225>] SyS_swapoff+0x215/0x600
> > > > [ 325.207287] [<ffffffff81434deb>] ? trace_hardirqs_on_thunk+0x17/0x19
> > > > [ 325.208300] [<ffffffff81890f6e>] system_call_fastpath+0x12/0x76
> > > > [ 325.209248] ---[ end trace 13f1014b56e5e711 ]---
> > > >
> > >
> > > Ok. What I think we need to do here is take a reference to the cl_xprt
> > > while holding the rcu_read_lock, and simply put it after we're done.
> > >
> > > That said...what happens if this xprt is switched out from under the
> > > clnt while we're swapping over it? It seems like
> > > rpc_switch_client_transport ought to be swap deactivating the old one
> > > and swap activating the new?
> > >
> > > Mel, any thoughts?
> > >
> >
> > Ok, I had a look at this code and this looks a little suspicious to me:
> >
> > ------------------[snip]--------------------
> > int xs_swapper(struct rpc_xprt *xprt, int enable)
> > {
> > struct sock_xprt *transport = container_of(xprt, struct sock_xprt,
> > xprt);
> > int err = 0;
> >
> > if (enable) {
> > xprt->swapper++;
> > xs_set_memalloc(xprt);
> > } else if (xprt->swapper) {
> > xprt->swapper--;
> > sk_clear_memalloc(transport->inet);
> > }
> >
> > return err;
> > }
> > ------------------[snip]--------------------
> >
> > There are a number of problems here, I think...
> >
>
> Sorry for the delay responding. I'm only intermittently available at the
> moment until mid next week.
>

No problem. None of this is terribly urgent, but since Jerome reported
the bug I thought I'd take a closer look.

> > 1) this is not done under a lock, so the non-atomic ++/-- is racy if
> > there are multiple swapons/swapoffs running concurrently on the same
> > xprt. Shouldn't those use an atomic?
> >
>
> It would be more appropriate to use atomics. It's a long time ago but I
> doubt I considered the possibility of multiple swapons racing at the
> time of implementation. Activation is typically a serialised task run
> from init.
>
> > 2) on enable, "swapper" is incremented and memalloc is set on the
> > socket. Do we need to do xs_set_memalloc every time swapon is called,
> > or only on a 0->1 swapper transition.
> >
>
> Every time because the static_key_slow_inc call is for the total number
> of connections.
>

That still seems wrong. The static_key would still be active even if
you just did it once per xprt.

> > 3) the !enable case also looks wrong. We decrement "swapper" and
> > then call sk_clear_memalloc, what if there are multiple swapfiles on
> > this xprt? Shouldn't that only be done when "swapper" goes to 0?
> >
>
> Hmm, that does sound correct. I don't think I was expecting multiple
> swap files per NFS mount although I did consider the possibility of
> multiple NFS mounts with a swapfile each.
>

Right, multiple nfs mounts that share a xprt and that have swapfile
each would have the same problem.

There's also the problem of an xprt being changed out from under the
clnt in the case of a NFSv4 migration event. The cl_xprt is rcu-managed
and it can basically change at any time.

I think the right thing to do is to keep a per-rpc_clnt count of
swapfiles, and then a per-xprt count of clnts that have swapfiles. Then
when you go to switch out the xprt, you could simply check to see if
the rpc_clnt has a non-zero counter and xs_set_memalloc the new socket
if so (and probably sk_clear_memalloc the old one).

There is still some raciness potential there if you have a migration
event occur while you're doing swapon/swapoff, but that's pretty
unlikely. Might could even do something like this in nfs_swap_activate:

bump client's counter
synchronize_rcu
rcu deref the clnt->cl_xprt
xprt_get while holding rcu_read_lock
call xs_swapper
xprt_put

--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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