Re: NFS/d_splice_alias breakage

Oleg Drokin
2016-06-10T16:49:30Z

On Jun 9, 2016, at 9:33 PM, Oleg Drokin wrote:

>
> On Jun 6, 2016, at 7:36 PM, Oleg Drokin wrote:
>
>> Well, I have some bad news.
>>
>> This patch does not fix the issue 100% of the time apparently, I just hit it again.
>
> Ok, so now finally a piece of good news.
> Whatever was causing this other much harder to hit crash is gone in -rc2, gone are
> some other disturbing things I saw.

Famous last words, I guess.
It all returned overnight.

But this time it's different from the past couple.

0xffffffff813aa8bb is in nfs4_do_open (/home/green/bk/linux/fs/nfs/nfs4proc.c:2482).
2477 d_drop(dentry);
2478 alias = d_exact_alias(dentry, state->inode);
2479 if (!alias)
2480 alias = d_splice_alias(igrab(state->inode), dentry);
2481 /* d_splice_alias() can't fail here - it's a non-directory */

So it appears the dentry that was negative turned positive in the middle of
d_exact_alias call… and also despite us doing d_drop(dentry), it's also already
hashed?
If this can happen in the middle of our operations here, same thing
presumably can happen in the other place we patched up in nfs_atomic_open -
we do d_drop there, but by the time we get into d_splice_alias it's now
all hashed again by some racing thread, that would explain
some of the rarer earlier crashes I had in -rc1 after the initial fix.

I wonder if it could be a remote-fs specific open vs open race?

Say we have atomic open for parent1/file1, this locks parent1 and proceeds to lookup
file1.
Now before the lookup commences, some other thread moves file1 to parent2
and yet some other thread goes to open parent2/file1.
Eventually it all converges with two threads trying to instantiate file1,
if we get it "just wrong" then a clash like what we see can happen, no?
Hm, I guess then both opens would have their own dentry and it's only inode that's
shared, so that cannot be it.
How can anything find a dentry we presumably just allocated and hash it while we
are not done with it, I wonder?
Also I wonder if all of this is somehow related to this other problem I am having
where drop_nlink reports going into negative territory on rename() call, I hit this
twice already and I guess I just need to convert that to BUG_ON instead for a
closer examination.

The dentry is (I finally have a crashdump):

crash> p *(struct dentry *)0xffff880055dbd2e8
$4 = {
d_flags = 4718796,
d_seq = {
sequence = 4,
dep_map = {
key = 0xffffffff8337b1c0 <__key.41115>,
class_cache = {0x0, 0x0},
name = 0xffffffff81c79c66 "&dentry->d_seq",
cpu = 6,
ip = 6510615555426900570
}
},
d_hash = {
next = 0x0,
pprev = 0xffffc900000fd9c0
},
d_parent = 0xffff8800079d1008,
d_name = {
{
{
hash = 2223188567,
len = 2
},
hash_len = 10813123159
},
name = 0xffff880055dbd358 "10"
},
d_inode = 0xffff880066d8ab38,
d_iname = "10\000ZZZZZZZZZZZZZZZZZZZZZZZZZZZZ",
d_lockref = {
{
{
lock = {
{
rlock = {
raw_lock = {
val = {
counter = 0
}
},
magic = 3735899821,
owner_cpu = 4294967295,
owner = 0xffffffffffffffff,
dep_map = {
key = 0xffffffff8337b1c8 <__key.41114>,
class_cache = {0xffffffff82c1c3a0 <lock_classes+47616>, 0x0},
name = 0xffffffff81c65bc8 "&(&dentry->d_lockref.lock)->rlock",
cpu = 3,
ip = 18446744071583760701
}
},
{
__padding = "\000\000\000\000╜N╜чЪЪЪЪZZZZЪЪЪЪЪЪЪЪ",
dep_map = {
key = 0xffffffff8337b1c8 <__key.41114>,
class_cache = {0xffffffff82c1c3a0 <lock_classes+47616>, 0x0},
name = 0xffffffff81c65bc8 "&(&dentry->d_lockref.lock)->rlock",
cpu = 3,
ip = 18446744071583760701
}
}
}
},
count = 3
}
}
},
d_op = 0xffffffff81a46780 <nfs4_dentry_operations>,
d_sb = 0xffff88004eaf3000,
d_time = 6510615555426956154,
d_fsdata = 0x0,
{
d_lru = {
next = 0xffff880066d7e3e8,
prev = 0xffff8800036736c8
},
d_wait = 0xffff880066d7e3e8
},
d_child = {
next = 0xffff8800268629b8,
prev = 0xffff8800079d1128
},
d_subdirs = {
next = 0xffff880055dbd408,
prev = 0xffff880055dbd408
},
d_u = {
d_alias = {
next = 0x0,
pprev = 0xffff880066d8ad20
},
d_in_lookup_hash = {
next = 0x0,
pprev = 0xffff880066d8ad20
},
d_rcu = {
next = 0x0,
func = 0xffff880066d8ad20
}
}
}

[22845.516232] kernel BUG at /home/green/bk/linux/fs/dcache.c:2989!
[22845.516864] invalid opcode: 0000 [#1] SMP
[22845.517350] Modules linked in: loop rpcsec_gss_krb5 acpi_cpufreq joydev tpm_tis tpm virtio_console i2c_piix4 pcspkr nfsd ttm drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm virtio_blk serio_raw floppy
[22845.519236] CPU: 0 PID: 2894259 Comm: cat Not tainted 4.7.0-rc2-vm-nfs+ #122
[22845.519843] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[22845.520259] task: ffff8800640d8e40 ti: ffff88004665c000 task.ti: ffff88004665c000
[22845.520975] RIP: 0010:[<ffffffff81287811>] [<ffffffff81287811>] d_splice_alias+0x1e1/0x470
[22845.521746] RSP: 0018:ffff88004665fa08 EFLAGS: 00010286
[22845.522122] RAX: ffff880066d8ab38 RBX: 0000000000000000 RCX: 0000000000000001
[22845.522524] RDX: 000000000000191a RSI: ffff880055dbd2e8 RDI: ffff880066d8ab38
[22845.522926] RBP: ffff88004665fa40 R08: 0000235352190a66 R09: ffff880055dbd2e8
[22845.523328] R10: 0000000000000001 R11: 0000000000000000 R12: ffff880072d43c00
[22845.523731] R13: ffff880072d43c00 R14: ffff88004883a580 R15: ffff880052615800
[22845.524131] FS: 00007f3b2c27d700(0000) GS:ffff88006b400000(0000) knlGS:0000000000000000
[22845.524846] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[22845.525225] CR2: 000055649285c0d0 CR3: 000000004d961000 CR4: 00000000000006f0
[22845.525628] Stack:
[22845.525964] ffff88004665fa20 ffffffff8188a1f7 0000000000000000 ffff880072d43c00
[22845.526700] ffff880072d43c00 ffff88004883a580 ffff880052615800 ffff88004665fb20
[22845.527433] ffffffff813aa8bb ffffffff00000000 ffff8800024000c0 ffff88004665fd80
[22845.528161] Call Trace:
[22845.528504] [<ffffffff8188a1f7>] ? _raw_spin_unlock+0x27/0x40
[22845.528943] [<ffffffff813aa8bb>] nfs4_do_open+0xaeb/0xb10
[22845.529331] [<ffffffff813aa9d0>] nfs4_atomic_open+0xf0/0x110
[22845.529718] [<ffffffff8137eefc>] nfs_atomic_open+0x1ac/0x700
[22845.530112] [<ffffffff8127900a>] lookup_open+0x2ea/0x770
[22845.530488] [<ffffffff8127bee5>] path_openat+0x6e5/0xc20
[22845.530881] [<ffffffff8104a425>] ? kvm_sched_clock_read+0x25/0x40
[22845.531268] [<ffffffff8127d981>] do_filp_open+0x91/0x100
[22845.531645] [<ffffffff8188a1f7>] ? _raw_spin_unlock+0x27/0x40
[22845.532027] [<ffffffff8128efc0>] ? __alloc_fd+0x100/0x200
[22845.532405] [<ffffffff81269f80>] do_sys_open+0x130/0x220
[22845.533157] [<ffffffff8126a08e>] SyS_open+0x1e/0x20

vmcore is at http://knox.linuxhacker.ru/tmp/dentry2/vmcore.gz
vmlinux is at http://knox.linuxhacker.ru/tmp/dentry2/vmlinux.gz