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Date: Thu, 26 Jan 2012 18:56:12 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Rusty Russell <rusty@rustcorp.com.au>, Tejun Heo <tj@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: + kmod-avoid-deadlock-by-recursive-kmod-call.patch added to
	-mm tree
Message-ID: <20120126175612.GA24011@redhat.com>
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> @@ -449,6 +460,16 @@ int call_usermodehelper_exec(struct subp
>  		retval = -EBUSY;
>  		goto out;
>  	}
> +	/*
> +	 * Worker thread must not wait for khelper thread at below
> +	 * wait_for_completion() if the thread was created with CLONE_VFORK
> +	 * flag, for khelper thread is already waiting for the thread at
> +	 * wait_for_completion() in do_fork().
> +	 */
> +	if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
> +		retval = -EBUSY;
> +		goto out;
> +	}

So, this is because khelper_wq's max_active == 1.

Can't we simply kill khelper_wq and use system_unbound_wq instead?

Oleg.