Re: [PATCH 6/8] trace_uprobe/sdt: Fix multiple update of same reference counter

[Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>]

On 03/14/2018 07:45 PM, Masami Hiramatsu wrote:
> On Tue, 13 Mar 2018 18:26:01 +0530
> Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com> wrote:
>
>> For tiny binaries/libraries, different mmap regions points to the
>> same file portion. In such cases, we may increment reference counter
>> multiple times. But while de-registration, reference counter will get
>> decremented only by once leaving reference counter > 0 even if no one
>> is tracing on that marker.
>>
>> Ensure increment and decrement happens in sync by keeping list of
>> mms in trace_uprobe. Increment reference counter only if mm is not
>> present in the list and decrement only if mm is present in the list.
>>
>> Example
>>
>>   # echo "p:sdt_tick/loop2 /tmp/tick:0x6e4(0x10036)" > uprobe_events
>>
>> Before patch:
>>
>>   # perf stat -a -e sdt_tick:loop2
>>   # /tmp/tick
>>   # dd if=/proc/`pgrep tick`/mem bs=1 count=1 skip=$(( 0x10020036 )) 2>/dev/null | xxd
>>    0000000: 02                                       .
>>
>>   # pkill perf
>>   # dd if=/proc/`pgrep tick`/mem bs=1 count=1 skip=$(( 0x10020036 )) 2>/dev/null | xxd
>>   0000000: 01                                       .
>>
>> After patch:
>>
>>   # perf stat -a -e sdt_tick:loop2
>>   # /tmp/tick
>>   # dd if=/proc/`pgrep tick`/mem bs=1 count=1 skip=$(( 0x10020036 )) 2>/dev/null | xxd
>>   0000000: 01                                       .
>>
>>   # pkill perf
>>   # dd if=/proc/`pgrep tick`/mem bs=1 count=1 skip=$(( 0x10020036 )) 2>/dev/null | xxd
>>   0000000: 00                                       .
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
>> ---
>>  kernel/trace/trace_uprobe.c | 105 +++++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 103 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
>> index b6c9b48..9bf3f7a 100644
>> --- a/kernel/trace/trace_uprobe.c
>> +++ b/kernel/trace/trace_uprobe.c
>> @@ -50,6 +50,11 @@ struct trace_uprobe_filter {
>>  	struct list_head	perf_events;
>>  };
>>  
>> +struct sdt_mm_list {
>> +	struct mm_struct *mm;
>> +	struct sdt_mm_list *next;
>> +};
> Oh, please use struct list_head instead of defining your own pointer-chain :(

Sure, will change it.

>> +
>>  /*
>>   * uprobe event core functions
>>   */
>> @@ -61,6 +66,8 @@ struct trace_uprobe {
>>  	char				*filename;
>>  	unsigned long			offset;
>>  	unsigned long			ref_ctr_offset;
>> +	struct sdt_mm_list		*sml;
>> +	struct rw_semaphore		sml_rw_sem;
> BTW, is there any reason to use rw_semaphore? (mutex doesn't fit?)

Hmm.. No specific reason.. will use a mutex instead.

Thanks for the review :)
Ravi