On Tue, Jun 29, 2010 at 07:18:46PM +0200, Oleg Nesterov wrote:
> On 06/29, Paul E. McKenney wrote:
> >
> > 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
I agree -- I cannot see how this wmb() can help.
> > [... snip a lot of good ideas ...]
>
> Thanks a lot, Paul ;)
Glad you liked them. ;-)
Thanx, Paul
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