Re: [PATCH 4/7] pid: modify task_tgid_nr to work without task->tgid.

On 14/02/20, Oleg Nesterov wrote: > 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. I thought I recently saw a statistic that there were only 7 instances of ->tgid, but a quick search comes up with >50 outside of audit!

> > - 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. So at the very least, I'd need static inline pid_t task_tgid_nr(struct task_struct *tsk) { return pid_nr(is_alive(tsk) ? task_tgid(tsk) : NULL); } And it sounds like I might even need this since the status of is_alive could change between the time I call it and the time I call task_tgid: static inline pid_t task_tgid_nr(struct task_struct *tsk) { pid_t pid; task_lock(&tsk); pid = pid_nr(is_alive(tsk) ? task_tgid(tsk) : NULL); task_unlock(&tsk); return pid; } Is task_lock() sufficient, or do I need the heavier read_lock(&tasklist_lock)?

> 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. Basically check for an inline error return of 0 signalling a failure rather than the idle task.