Re: [PATCH 1/1] perf_event: add check for group_list if the parent isn't counted

[Wang Liming <liming.wang@windriver.com>]

Peter Zijlstra wrote:
> On Wed, 2009-12-30 at 19:28 +0800, Liming Wang wrote:
>> If the parent has no entry in group_list, child_ctx will not be
>> allocated, which will lead dereference of a NULL child_ctx.
> 
> That changelog sucks.
Sorry the changelog is too simple.

> 
> Best I can make of it is that there is a race where the parent gets his
> context instantiated and we manage to get the mutex before the other
> thread manages to add the first event.
> 
> Then we observe parent_event_ctx but have an empty list.
> 
> Is that it?
I didn't find this case.
In my case, if I perf record a existing process with "--pid" and finish record,
and if later the recorded process forks a process, the condition will occur.

Liming Wang
> 
> In that case, would it not be better to change the if (inherited_all)
> condition to if (inherited_all && child_ctx) ?
> 
>> Signed-off-by: Liming Wang <liming.wang@windriver.com>
>> ---
>>  kernel/perf_event.c |    2 ++
>>  1 files changed, 2 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
>> index 5b987b4..3664c4b 100644
>> --- a/kernel/perf_event.c
>> +++ b/kernel/perf_event.c
>> @@ -5126,6 +5126,8 @@ int perf_event_init_task(struct task_struct *child)
>>  	 */
>>  	mutex_lock(&parent_ctx->mutex);
>>  
>> +	if (list_empty(&parent_ctx->group_list))
>> +		goto exit;
>>  	/*
>>  	 * We dont have to disable NMIs - we are only looking at
>>  	 * the list, not manipulating it:
> 
> 
>