On 06/29, Paul E. McKenney wrote:
quoted text
>
> On Tue, Jun 29, 2010 at 03:31:31PM +0200, Oleg Nesterov wrote:
>
> > So, afaics, set_task_comm()->wmb() buys nothing and should be removed.
> > The last zero char in task_struct->comm[] is always here, at least this
> > guarantees that strcpy(char *dest, tsk->comm) is always safe.
> >
> > (I cc'ed the expert, Paul can correct me)
>
> First, all of the implementations that I can see do task_lock(tsk), which
> should prevent readers from seeing any changes.  So I am guessing that
> you guys want to allow readers to get at ->comm without having to acquire
> this lock.

Ah, sorry for confusion.

No, we are not trying to invent the lockless get_task_comm(). I'd say
it is not needed, if we really care about the precise ->comm we can
take task->alloc_lock.

The only problem is that I believe that set_task_comm() wrongly pretends
wmb() can help the lockless reader, it does:

	task_lock(tsk);

	/*
	 * Threads may access current->comm without holding
	 * the task lock, so write the string carefully.
	 * Readers without a lock may see incomplete new
	 * names but are safe from non-terminating string reads.
	 */
	memset(tsk->comm, 0, TASK_COMM_LEN);
	wmb();
	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
	task_unlock(tsk);

but afaics this wmb() buys absolutely nothing if we race with the
reader doing, say,

	printk("my name is %s\n", current->comm);

Afaics, this wmb()

	- can't prevent from printing the mixture of the old/new data

	- is not needed to make strcpy(somewhere, task->comm) safe,
	  the final char is always '0', we never change it.

	- adds the unnecessary confusion

quoted text
> [... snip a lot of good ideas ...]

Thanks a lot, Paul ;)

Oleg.

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