Andrew Morton <[EMAIL PROTECTED]> writes:
> 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?
Well I took a quick look.
Yeah this looks complex.
Mutation of the hash table is protected by pidmap_lock.
But attachments of tasks to hash entries is protected task_lock.
And it looks like it has been that way since commit
92476d7fc0326a409ab1d3864a04093a6be9aca7
I thought free_pid did not have any requirements that a lock be held when
it was called, taking all of the needed locks.
Now how read_lock doesn't imply rcu_read_lock is another question.
Anyway I have to run. I will see about looking at this in a bit.
Eric
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