Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a ring buffer

["zhangwei(Jovi)" <jovi.zhangwei@huawei.com>]

On 2013/8/23 0:42, Steven Rostedt wrote:
> On Fri, 09 Aug 2013 18:56:54 +0900
> Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> 
>> (2013/08/09 17:45), Namhyung Kim wrote:
>>> From: Namhyung Kim <namhyung.kim@lge.com>
>>>
>>> Fetching from user space should be done in a non-atomic context.  So
>>> use a temporary buffer and copy its content to the ring buffer
>>> atomically.
>>>
>>> While at it, use __get_data_size() and store_trace_args() to reduce
>>> code duplication.
>>
>> I just concern using kmalloc() in the event handler. For fetching user
>> memory which can be swapped out, that is true. But most of the cases,
>> we can presume that it exists on the physical memory.
>>
> 
> 
> What about creating a per cpu buffer when uprobes are registered, and
> delete them when they are finished? Basically what trace_printk() does
> if it detects that there are users of trace_printk() in the kernel.
> Note, it does not deallocate them when finished, as it is never
> finished until reboot ;-)
> 
> -- Steve
>
I also thought out this approach, but the issue is we cannot fetch user
memory into per-cpu buffer, because use per-cpu buffer should under
preempt disabled, and fetching user memory could sleep.

jovi.