linux-kernel - Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of get/put_task

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Message-ID: <1c0c57da-6b2b-f12d-f397-4ddd467bb57c@gmail.com>
Date:   Mon, 21 Aug 2023 13:38:43 -0700
From:   Kui-Feng Lee <sinquersw@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>, Yonghong Song <yhs@...com>,
        Kui-Feng Lee <kuifeng@...com>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of
 get/put_task_struct



On 8/21/23 13:32, Kui-Feng Lee wrote:
> 
> 
> On 8/21/23 13:03, Oleg Nesterov wrote:
>> get_pid_task() makes no sense, the code does put_task_struct() soon 
>> after.
>> Use find_task_by_pid_ns() instead of find_pid_ns + get_pid_task and kill
>> kill put_task_struct(), this allows to do get_task_struct() only once
>> before return.
>>
>> While at it, kill the unnecessary "if (!pid)" check in the "if (!*tid)"
>> block, this matches the next usage of find_pid_ns() + get_pid_task() in
>> this function.
>>
>> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>> ---
>>   kernel/bpf/task_iter.c | 12 ++----------
>>   1 file changed, 2 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 4d1125108014..1589ec3faded 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -42,9 +42,6 @@ static struct task_struct 
>> *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>>       if (!*tid) {
>>           /* The first time, the iterator calls this function. */
>>           pid = find_pid_ns(common->pid, common->ns);
>> -        if (!pid)
>> -            return NULL;
>> -
>>           task = get_pid_task(pid, PIDTYPE_TGID);
>>           if (!task)
>>               return NULL;
>> @@ -66,17 +63,12 @@ static struct task_struct 
>> *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>>           return task;
>>       }
>> -    pid = find_pid_ns(common->pid_visiting, common->ns);
>> -    if (!pid)
>> -        return NULL;
>> -
>> -    task = get_pid_task(pid, PIDTYPE_PID);
>> +    task = find_task_by_pid_ns(common->pid_visiting, common->ns);
>>       if (!task)
>>           return NULL;
>>   retry:
>>       next_task = next_thread(task);
>> -    put_task_struct(task);
> 
> It called get_task_struct() against this task to hold a refcount at the
> previous time calling this function. When will it release the refcount?


Oh! I missed the fact that the caller will handle it.

> 
>>       saved_tid = *tid;
>>       *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
>> @@ -88,7 +80,6 @@ static struct task_struct 
>> *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>>           return NULL;
>>       }
>> -    get_task_struct(next_task);
>>       common->pid_visiting = *tid;
>>       if (skip_if_dup_files && task->files == 
>> task->group_leader->files) {
>> @@ -96,6 +87,7 @@ static struct task_struct 
>> *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>>           goto retry;
>>       }
>> +    get_task_struct(next_task);
>>       return next_task;
>>   }