On Tue, 29 Jan 2008 19:40:19 +0300
Oleg Nesterov <[EMAIL PROTECTED]> wrote:
> With CONFIG_PREEMPT_RCU read_lock(tasklist_lock) doesn't imply
> rcu_read_lock(),
> but find_pid_ns()->hlist_for_each_entry_rcu() should be safe under tasklist.
>
> Usually it is, detach_pid() is always called under write_lock(tasklist_lock),
> but copy_process() calls free_pid() lockless.
>
> "#ifdef CONFIG_PREEMPT_RCU" is added mostly as documentation, perhaps it is
> too ugly and should be removed.
>
> Signed-off-by: Oleg Nesterov <[EMAIL PROTECTED]>
>
> --- MM/kernel/fork.c~PR_RCU 2008-01-27 17:09:47.000000000 +0300
> +++ MM/kernel/fork.c 2008-01-29 19:23:44.000000000 +0300
> @@ -1335,8 +1335,19 @@ static struct task_struct *copy_process(
> return p;
>
> bad_fork_free_pid:
> - if (pid != &init_struct_pid)
> + if (pid != &init_struct_pid) {
> +#ifdef CONFIG_PREEMPT_RCU
> + /*
> + * read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> + * make sure find_pid() is safe under read_lock(tasklist).
> + */
> + write_lock_irq(&tasklist_lock);
> +#endif
> free_pid(pid);
> +#ifdef CONFIG_PREEMPT_RCU
> + write_unlock_irq(&tasklist_lock);
> +#endif
> + }
> bad_fork_cleanup_namespaces:
> exit_task_namespaces(p);
> bad_fork_cleanup_keys:
My attempt to understand this change timed out.
kernel/pid.c is full of global but undocumented functions. What are the
locking requirements for free_pid()? free_pid_ns()? If it's just
caller-must-hold-rcu_read_lock() then why not use rcu_read_lock() here?
If the locking is "caller must hold write_lock_irq(tasklist_lock) then the
sole relevant comment in there (in free_pid()) is wrong.
Guys, more maintainable code please?
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