linux-kernel - Re: [PATCH v3 2/3] hvc_init(): Enforce one-time initialization.

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Date:	Wed, 09 Nov 2011 18:24:12 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Miche Baker-Harvey <miche@...gle.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Anton Blanchard <anton@...ba.org>,
	Amit Shah <amit.shah@...hat.com>,
	Mike Waychison <mikew@...gle.com>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Eric Northrup <digitaleric@...gle.com>
Subject: Re: [PATCH v3 2/3] hvc_init():  Enforce one-time initialization.

On Tue, 2011-11-08 at 13:45 -0800, Miche Baker-Harvey wrote:
> hvc_init() must only be called once, and no thread should continue with hvc_alloc()
> until after initialization is complete.  The original code does not enforce either
> of these requirements.  A new mutex limits entry to hvc_init() to a single thread,
> and blocks all later comers until it has completed.
> 
> This patch fixes multiple crash symptoms.

Hi Miche,

A few nit-picky comments below ..

> @@ -84,6 +85,10 @@ static LIST_HEAD(hvc_structs);
>   * list traversal.
>   */
>  static DEFINE_SPINLOCK(hvc_structs_lock);
> +/*
> + * only one task does allocation at a time.
> + */
> +static DEFINE_MUTEX(hvc_ports_mutex);

The comment is wrong, isn't it? Only one task does _init_ at a time.
Once the driver is initialised allocs can run concurrently.

So shouldn't it be called hvc_init_mutex ?

> @@ -825,11 +830,15 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
>  	int i;
>  
>  	/* We wait until a driver actually comes along */
> +	mutex_lock(&hvc_ports_mutex);
>  	if (!hvc_driver) {
>  		int err = hvc_init();
> -		if (err)
> +		if (err) {
> +			mutex_unlock(&hvc_ports_mutex);
>  			return ERR_PTR(err);
> +		}
>  	}
> +	mutex_unlock(&hvc_ports_mutex);
>  
>  	hp = kzalloc(ALIGN(sizeof(*hp), sizeof(long)) + outbuf_size,
>  			GFP_KERNEL);

It'd be cleaner I think to do all the locking in hvc_init(). That's safe
as long as you recheck !hvc_driver in hvc_init() with the lock held.

cheers


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