Re: [RFC] ps command race fix

[KAMEZAWA Hiroyuki]

On Mon, 24 Jul 2006 18:20:00 -0700
Andrew Morton <akpm@xxxxxxxx> wrote:
> 
> It allocates a potentially-significant amount of memory per-task, until
> that tasks exits (we could release it earlier, but the problem remains) and
> it adds yet another global lock in the process exit path.
> 
I see.

> >  5 files changed, 138 insertions(+), 62 deletions(-)
> 
> And it adds complexity and code.
> 
> So I think we're still seeking a solution to this.
> 
> Options might be:
> 
> a) Pin the most-recently-visited task in some manner, so that it is
>    still on the global task list when we return.  That's fairly simple to
>    do (defer the release_task()) but it affects task lifetime and visibility
>    in rare and worrisome ways.
> 
> b) Change proc_pid_readdir() so that it walks the pid_hash[] array
>    instead of the task list.  Need to do something clever when traversing
>    each bucket's list, but I'm not sure what ;) It's the same problem.
> 
>    Possibly what we could do here is to permit the task which is walking
>    /proc to pin a particular `struct pid': take a ref on it then when we
>    next start walking one of the pid_hash[] chains, we _know_ that the
>    `struct pid' which we're looking for will still be there.  Even if it
>    now refers to a departed process.
> 
> c) Nuke the pid_hash[], convert the whole thing to a radix-tree. 
>    They're super-simple to traverse.  Not sure what we'd index it by
>    though.
> 
> I guess b) is best.
> 

I tried b) at the first place. but it was not very good because 
proc_pid_readdir() has to traverse all pids, not tgids. So, I had to access
task_struct of the pid. I wanted to avoid to access task struct itself,
my patch implemented a table made only from tgids.

But as you say, my patch is much intrusive. 
I'll dig this more. thank you for advise.

Thanks,
-Kame




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