Re: 2.6.26-rc4: RIP find_pid_ns+0x6b/0xa0

[Oleg Nesterov <oleg@tv-sign.ru>]

On 05/27, Alexey Dobriyan wrote:
>
> PREEMPT_RCU is in use, again. And die counter is 2 because of CFQ oops
> I haven't noticed earlier.
> 
> 0xffffffff802447cb is in find_pid_ns (kernel/pid.c:297).
> 292             struct hlist_node *elem;
> 293             struct upid *pnr;
> 294
> 295             hlist_for_each_entry_rcu(pnr, elem,
> 296                             &pid_hash[pid_hashfn(nr, ns)], pid_chain)
> 297                     if (pnr->nr == nr && pnr->ns == ns)
> 298                             return container_of(pnr, struct pid,
> 299                                             numbers[ns->level]);
> 300
> 301             return NULL;
> 
> 
> general protection fault: 0000 [2] PREEMPT SMP DEBUG_PAGEALLOC
> CPU 0 
> Modules linked in: ext2 nf_conntrack_irc xt_state iptable_filter ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack ip_tables x_tables usblp ehci_hcd uhci_hcd usbcore sr_mod cdrom
> Pid: 15599, comm: profil01 Tainted: G      D   2.6.26-rc4 #1
> RIP: 0010:[<ffffffff802447cb>]  [<ffffffff802447cb>] find_pid_ns+0x6b/0xa0
> RSP: 0018:ffff810129021ea8  EFLAGS: 00010202
> RAX: ffff810130580948 RBX: 0000000000003cef RCX: ffff81017d865278
> RDX: 6b6b6b6b6b6b6b6b RSI: ffffffff80566760 RDI: 0000000000003cef
> RBP: ffff810129021ea8 R08: 0000000000000000 R09: 00007f9a93987b70
> R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
> R13: 0000000000000011 R14: 0000000000000000 R15: 0000000000000000
> FS:  00007f9a9397c6f0(0000) GS:ffffffff805c6000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 000000000257f2e8 CR3: 0000000102479000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process profil01 (pid: 15599, threadinfo ffff810129020000, task ffff81004bc24500)
> Stack:  ffff810129021eb8 ffffffff8024487d ffff810129021f78 ffffffff8023f275
>  0000000000000011 0000000000000000 0000000000003cef ffff810129020000
>  ffffffff8061b140 00007f9a93989bc0 00007fff9b98a410 ffffffff8045fd63
> Call Trace:
>  [<ffffffff8024487d>] find_vpid+0x1d/0x20
>  [<ffffffff8023f275>] sys_kill+0x85/0x1b0

Is this reproducible?

In theory find_pid() is not safe without rcu_read_lock() if CONFIG_PREEMPT_RCU.
But we have a lot of "read_lock(tasklist_lock) + find_pid()", this was legal
and documented. It was actually broken, but happened to work because read_lock()
implied rcu_read_lock().

Could you look at

	[PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU
	http://marc.info/?t=120162615300012

?

I am not sure this is the actual reason though, the race is very unlikely.

Oleg.