Re: [PATCH] mm-add-vfree_atomic-fix

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 12/13/2016 08:24 PM, Michal Hocko wrote:
> On Tue 13-12-16 08:57:34, Andy Lutomirski wrote:
>> On Tue, Dec 13, 2016 at 2:12 AM, Michal Hocko <mhocko@kernel.org> wrote:
>>> [CC Andy]
>>>
>>> I've noticed the same
>>> http://lkml.kernel.org/r/20161209142820.GA4334@dhcp22.suse.cz
>>> and also concluded same as you
>>>
>>> On Mon 12-12-16 17:46:21, Andrey Ryabinin wrote:
>>>> DEBUG_PREEMPT complains about using this_cpu_ptr() in preemptible:
>>>>       BUG: using smp_processor_id() in preemptible [00000000] code: iperf-300s-cs-l/277
>>>>       caller is debug_smp_processor_id+0x17/0x19
>>>>       CPU: 1 PID: 277 Comm: iperf-300s-cs-l Not tainted 4.9.0-rc8-00140-gcc639db #2
>>>>        ffffc900003f3cf0 ffffffff8123ae6f 0000000000000001 ffffffff818181da
>>>>        ffffc900003f3d20 ffffffff81252f41 0000000000012de0 00000000fffffdff
>>>>        ffff880009328f40 ffff88000592c400 ffffc900003f3d30 ffffffff81252f6a
>>>>       Call Trace:
>>>>        [<ffffffff8123ae6f>] dump_stack+0x9a/0xd0
>>>>        [<ffffffff81252f41>] check_preemption_disabled+0xdd/0xef
>>>>        [<ffffffff81252f6a>] debug_smp_processor_id+0x17/0x19
>>>>        [<ffffffff811796df>] __vfree_deferred+0x16/0x4c
>>>>        [<ffffffff8117b584>] vfree_atomic+0x22/0x24
>>>>        [<ffffffff81094f5d>] free_thread_stack+0xc2/0x106
>>>>        [<ffffffff810951be>] put_task_stack+0x4c/0x62
>>>>        [<ffffffff81095f81>] copy_process+0x7e0/0x16e8
>>>>        [<ffffffff8109702d>] _do_fork+0xbb/0x2d3
>>>>        [<ffffffff810465e8>] ? __do_page_fault+0x2e1/0x384
>>>>        [<ffffffff8112633f>] ? trace_hardirqs_off_caller+0x12/0x24
>>>>        [<ffffffff810972cb>] SyS_clone+0x19/0x1b
>>>>        [<ffffffff81003800>] do_syscall_64+0x143/0x173
>>>>        [<ffffffff81507289>] entry_SYSCALL64_slow_path+0x25/0x25
>>>>
>>>> Use raw_cpu_ptr() instead of this_cpu_ptr() to hide this warning.
>>>> It's fine because llist_add() implementation is lock-less, so it works even
>>>> if we adding to the list of some other cpu. schedule_work() is also preempt-safe.
>>>>
>>>> Reported-by: kernel test robot <ying.huang@linux.intel.com>
>>>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>>>
>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>
>> But not quite acked by me.  What happened to the vfree code that
>> causes vfree_deferred to be called in a preemptable context?  That
>> sounds like a bug.
> 
> Not sure I understand but the above stack points to a preemptible
> context (copy_process). My stack was different and it looks preemptible as well.
> free_thread_stack calls vfree_atomic unconditionally. So I am not sure
> why do you think this is a bug?
> 
>> (This code doesn't exist in Linus' tree.  What tree does this apply to.)
> 
> Anyway, now that I am looking at Andrew's tree I can see [1] which
> doesn't have this_cpu_ptr. So I am not sure where this this_cpu_ptr came
> from. Maybe the previous version of the patch which has shown up in the
> linux-next and Andrew has picked up [2] in the meantime. /me confused
> 

this_cpu_ptr() comes from the original patch http://lkml.kernel.org/r/1479474236-4139-5-git-send-email-hch@lst.de
Andrew picked up [2] and folded-merged it into original patch and sent it to Linus.
Now it in Linus tree, commit bf22e37a641327e34681b7b6959d9646e3886


> [1] http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-add-vfree_atomic.patch
> [2] http://lkml.kernel.org/r/1481553981-3856-1-git-send-email-aryabinin@virtuozzo.com
>