Re: (mem hotplug, pcpu_alloc) BUG: sleeping function called from invalid context at kernel/mutex.c:94

[Tejun Heo <tj@kernel.org>]

Hello,

On 11/12/2010 07:56 PM, Alok Kataria wrote:
> We have seen following might_sleep warning while hot adding memory...
> 
> [  142.339267] BUG: sleeping function called from invalid context at kernel/mutex.c:94
> [  142.339276] in_atomic(): 0, irqs_disabled(): 1, pid: 4, name: migration/0
> [  142.339283] Pid: 4, comm: migration/0 Not tainted 2.6.35.6-45.fc14.x86_64 #1
> [  142.339288] Call Trace:
> [  142.339305]  [<ffffffff8103d12b>] __might_sleep+0xeb/0xf0
> [  142.339316]  [<ffffffff81468245>] mutex_lock+0x24/0x50
> [  142.339326]  [<ffffffff8110eaa6>] pcpu_alloc+0x6d/0x7ee
> [  142.339336]  [<ffffffff81048888>] ? load_balance+0xbe/0x60e
> [  142.339343]  [<ffffffff8103a1b3>] ? rt_se_boosted+0x21/0x2f
> [  142.339349]  [<ffffffff8103e1cf>] ? dequeue_rt_stack+0x18b/0x1ed
> [  142.339356]  [<ffffffff8110f237>] __alloc_percpu+0x10/0x12
> [  142.339362]  [<ffffffff81465e22>] setup_zone_pageset+0x38/0xbe
> [  142.339373]  [<ffffffff810d6d81>] ? build_zonelists_node.clone.58+0x79/0x8c
> [  142.339384]  [<ffffffff81452539>] __build_all_zonelists+0x419/0x46c
> [  142.339395]  [<ffffffff8108ef01>] ? cpu_stopper_thread+0xb2/0x198
> [  142.339401]  [<ffffffff8108f075>] stop_machine_cpu_stop+0x8e/0xc5
> [  142.339407]  [<ffffffff8108efe7>] ? stop_machine_cpu_stop+0x0/0xc5
> [  142.339414]  [<ffffffff8108ef57>] cpu_stopper_thread+0x108/0x198
> [  142.339420]  [<ffffffff81467a37>] ? schedule+0x5b2/0x5cc
> [  142.339426]  [<ffffffff8108ee4f>] ? cpu_stopper_thread+0x0/0x198
> [  142.339434]  [<ffffffff81065f29>] kthread+0x7f/0x87
> [  142.339443]  [<ffffffff8100aae4>] kernel_thread_helper+0x4/0x10
> [  142.339449]  [<ffffffff81065eaa>] ? kthread+0x0/0x87
> [  142.339455]  [<ffffffff8100aae0>] ? kernel_thread_helper+0x0/0x10
> [  142.340099] Built 5 zonelists in Node order, mobility grouping on.  Total pages: 289456
> [  142.340108] Policy zone: Normal
> 
> 
> This warning was seen on the FC14 kernel, though looking at the current
> git, the problem seems to exist on mainline too.
> The problem is that pcpu_alloc expects that it is called from non-atomic
> context as a result it grabs the pcpu_alloc_mutex. 
> In the memory-hotplug case though, we do end up calling pcpu_alloc from
> atomic context, while all cpus are stopped.
> 
> void build_all_zonelists(void *data)
> {
>    set_zonelist_order();
> 
>    if (system_state == SYSTEM_BOOTING) {
>       __build_all_zonelists(NULL);
>       mminit_verify_zonelist();
>       cpuset_init_current_mems_allowed();
>    } else {
>       /* we have to stop all cpus to guarantee there is no user
>          of zonelist */
>       stop_machine(__build_all_zonelists, data, NULL);   <=========
>       /* cpuset refresh routine should be here */
>    }
> 
> __build_all_zonelists eventually calls pcpu_alloc. 
> 
> I didn't dive through the history, so am not sure when was this
> regression introduced, but could have regressed with the new pcpu memory
> allocator.

Meh... the percpu allocator required user context from the beginning.
The new allocator didn't change that.

Wouldn't it be possible to prepare hotplug outside of cpu_stop and use
stop_machine() only to make it available to the system.  In general,
it's a very bad idea to allocate memory from inside stop_machine.  The
whole machine is stopped, after all.  In general, it shouldn't be too
difficult to add new resource without stop_machine too unlike removing
one.  Pekka, Christoph, any ideas?

Thanks.

-- 
tejun

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