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

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Message-Id: <1e51042b-d05b-dd55-82e6-818bb5be03d1@linux.vnet.ibm.com>
Date:   Wed, 14 Mar 2018 20:45:14 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     oleg@...hat.com, peterz@...radead.org, srikar@...ux.vnet.ibm.com,
        acme@...nel.org, ananth@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, alexander.shishkin@...ux.intel.com,
        alexis.berlemont@...il.com, corbet@....net,
        dan.j.williams@...el.com, gregkh@...uxfoundation.org,
        huawei.libin@...wei.com, hughd@...gle.com, jack@...e.cz,
        jglisse@...hat.com, jolsa@...hat.com, kan.liang@...el.com,
        kirill.shutemov@...ux.intel.com, kjlx@...pleofstupid.com,
        kstewart@...uxfoundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        milian.wolff@...b.com, mingo@...hat.com, namhyung@...nel.org,
        naveen.n.rao@...ux.vnet.ibm.com, pc@...ibm.com,
        pombredanne@...b.com, rostedt@...dmis.org, tglx@...utronix.de,
        tmricht@...ux.vnet.ibm.com, willy@...radead.org,
        yao.jin@...ux.intel.com, fengguang.wu@...el.com,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Subject: Re: [PATCH 6/8] trace_uprobe/sdt: Fix multiple update of same
 reference counter



On 03/14/2018 07:45 PM, Masami Hiramatsu wrote:
> On Tue, 13 Mar 2018 18:26:01 +0530
> Ravi Bangoria <ravi.bangoria@...ux.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@...ux.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