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    SubjectRe: [PATCH 4/7] pid: modify task_tgid_nr to work without task->tgid.
    On 01/23, Richard Guy Briggs wrote:
    >
    > task->tgid is an error prone construct and results in duplicate maintenance.
    > Start it's demise by modifying task_tgid_nr to not use it.

    Well, I disagree.

    Yes I agree that ->tgid should probably die. But this change itself doesn't
    help, it only makes task_tgid_nr() slower. We need to convert other users
    first, then consider this change along with ->tgid removal.

    Besides, this is not that simple and the patch doesn't look right:

    > static inline pid_t task_tgid_nr(struct task_struct *tsk)
    > {
    > - return tsk->tgid;
    > + return pid_nr(task_tgid(tsk));
    > }

    And what protect task_tgid? This is racy.

    The race is very unlikely, pid_nr() will likely hit pid == NULL if tsk
    exits. But still it can use the freed/unmapped/reused memory.

    And even if we add rcu_read_lock() the patch will add the semantics change,
    task_tgid_nr() can return 0 if tsk has already exited. At least this should
    be documented, but you also need to audit the users.

    Oleg.



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