Re: Soft lockups during reading /proc/PID/smaps

[David Rientjes]

On Thu, 31 Jul 2014, Aleksei Besogonov wrote:

> I'm getting weird soft lockups while reading smaps on loaded systems with
> some background cgroups usage. This issue can be reproduced with the most
> recent kernel.
> 
> Here's the stack trace:
> [ 1748.312052] BUG: soft lockup - CPU#6 stuck for 23s! [python2.7:1857]
> [ 1748.312052] Modules linked in: xfs xt_addrtype xt_conntrack
> iptable_filter ipt_MASQUERADE iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4
> nf_nat_ipv4 nf_nat nf_conntrack ip_tables x_tables bridge stp llc
> dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio libcrc32c nfsd
> auth_rpcgss nfs_acl nfs lockd sunrpc fscache dm_crypt psmouse serio_raw
> ppdev parport_pc i2c_piix4 parport xen_fbfront fb_sys_fops syscopyarea
> sysfillrect sysimgblt mac_hid isofs raid10 raid456 async_memcpy
> async_raid6_recov async_pq async_xor async_tx xor raid6_pq raid1 raid0
> multipath linear crct10dif_pclmul crc32_pclmul ghash_clmulni_intel
> aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd floppy
> [ 1748.312052] CPU: 6 PID: 1857 Comm: python2.7 Not tainted
> 3.15.5-031505-generic #201407091543

This isn't the most recent kernel, we're at 3.16-rc7 now, but I don't 
think there are any changes that would prevent this.

> [ 1748.312052] Hardware name: Xen HVM domU, BIOS 4.2.amazon 01/24/2014
> [ 1748.312052] task: ffff8800eab41930 ti: ffff8803b9a94000 task.ti:
> ffff8803b9a94000
> [ 1748.312052] RIP: 0010:[<ffffffff81013111>]  [<ffffffff81013111>]
> KSTK_ESP+0x11/0x40
> [ 1748.312052] RSP: 0018:ffff8803b9a97c68  EFLAGS: 00000287
> [ 1748.312052] RAX: 0000000000000000 RBX: ffff8803b60de3aa RCX: 00007f49ec000000
> [ 1748.312052] RDX: 0000000000000001 RSI: ffff8800eba1c730 RDI: ffff880399434b90
> [ 1748.312052] RBP: ffff8803b9a97c68 R08: 000000000000000a R09: 000000000000fffe
> [ 1748.312052] R10: 0000000000000000 R11: 0000000000000007 R12: 0000000000000001
> [ 1748.312052] R13: ffff8803b7a74108 R14: 00007f49ec021000 R15: ffff8803b9a9ffff
> [ 1748.312052] FS:  00007fcc9562b740(0000) GS:ffff8803cfcc0000(0000)
> knlGS:0000000000000000
> [ 1748.312052] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1748.312052] CR2: 00007fcc955ed180 CR3: 00000003b97a5000 CR4: 00000000001406e0
> [ 1748.312052] Stack:
> [ 1748.312052]  ffff8803b9a97ca8 ffffffff8117bcc9 ffff8803b9a97df8
> ffff880399435030
> [ 1748.312052]  ffff88003684d000 ffff8800eba1c730 0000000000000000
> ffff8803b5dfc980
> [ 1748.312052]  ffff8803b9a97d38 ffffffff81236612 ffff88030000002d
> 00007f4900000070
> [ 1748.312052] Call Trace:
> [ 1748.312052]  [<ffffffff8117bcc9>] vm_is_stack+0x59/0xe0
> [ 1748.312052]  [<ffffffff81236612>] show_map_vma+0x212/0x280
> [ 1748.312052]  [<ffffffff81236805>] show_smap+0x85/0x250
> [ 1748.312052]  [<ffffffff81237bc0>] ? smaps_pte_entry.isra.21+0x220/0x220
> [ 1748.312052]  [<ffffffff81236a03>] show_pid_smap+0x13/0x20
> [ 1748.312052]  [<ffffffff811f6016>] seq_read+0x256/0x3e0
> [ 1748.312052]  [<ffffffff811d30e1>] vfs_read+0xb1/0x180
> [ 1748.312052]  [<ffffffff811d335f>] SyS_read+0x4f/0xb0
> [ 1748.312052]  [<ffffffff8178527f>] tracesys+0xe1/0xe6

The while_each_thread() in vm_is_stack() looks suspicious since the task 
isn't current and rcu won't protect the iteration, and we also don't hold 
sighand lock or a readlock on tasklist_lock.

I think Oleg will know how to proceed, cc'd.
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