linux-kernel - Re: [PATCH 5/6] kmod: Add new call_usermodehelper

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Date:	Tue, 27 Mar 2012 17:43:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-security-module@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	NFS list <linux-nfs@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	"Bhamare, Sachin" <sbhamare@...asas.com>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Kay Sievers <kay.sievers@...y.org>,
	James Morris <jmorris@...ei.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"keyrings@...ux-nfs.org" <keyrings@...ux-nfs.org>
Subject: Re: [PATCH 5/6] kmod: Add new call_usermodehelper_timeout() API

Hi Boaz,

I'll read this series tomorrow, but

On 03/26, Boaz Harrosh wrote:
>
>  int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
>  {
>  	DECLARE_COMPLETION_ONSTACK(done);
> +	int wait_state;
>  	int retval = 0;
>
>  	helper_lock();
> @@ -540,19 +541,15 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
>  	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
>  		goto unlock;
>
> -	if (wait & UMH_KILLABLE) {
> -		retval = wait_for_completion_killable(&done);
> -		if (!retval)
> -			goto wait_done;
> -
> +	wait_state = (wait & UMH_KILLABLE) ? TASK_KILLABLE : 0;
> +	retval = wait_for_completion_timeout_state(&done, sub_info->timeout,
> +						   wait_state);
> +	if (unlikely(retval)) {
>  		/* umh_complete() will see NULL and free sub_info */
>  		if (xchg(&sub_info->complete, NULL))
>  			goto unlock;
> -		/* fallthrough, umh_complete() was already called */
>  	}
>
> -	wait_for_completion(&done);

at first glance this looks certainly wrong, or I misread the patch.

We can't remove the "fallback to wait_for_completion" logic until
you move the completion into subprocess_info (the next patch seems
to do this).

xchg() can race with umh_complete(). If it returns NULL, umh_complete()
was already called and got ->complete != NULL, we must not return until
umh_complete() finishes complete().

Oleg.

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