linux-kernel - Re: [PATCH] [workqueue] check values of pwq and wq in print_worker

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Date:	Wed, 02 Oct 2013 00:34:53 +0200
From:	Helge Deller <deller@....de>
To:	Tejun Heo <tj@...nel.org>
CC:	Libin <huawei.libin@...wei.com>, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [PATCH] [workqueue] check values of pwq and wq in print_worker_info()
 before use

On 10/01/2013 11:07 PM, Tejun Heo wrote:
> On Tue, Oct 01, 2013 at 05:03:48PM -0400, Tejun Heo wrote:
>> On Tue, Oct 01, 2013 at 10:53:31PM +0200, Helge Deller wrote:
>>> So, in summary my patch here is not really necessary, but for the sake of
>>> clean code I think it doesn't hurt either and as such it would be nice if
>>> you could apply it.
>>
>> What? function *must* take any value and try to access it and not
>> cause failure.  That's the *whole* purpose of that interface.  How is
>> having incomplete spurious checks around it "clean code" in any sense
>> of the word?  That doesn't make any sense.
> 
> Just in case you didn't know already.  probe_kernel_read()'s role is
> to take any ulong value and dereference it if it can.  If not, it can
> return any value, but it shouldn't crash in any case.  If you're just
> adding NULL test in probe_kernel_read(), you're just masking a common
> failure pattern and the kernel still *will* panic while dumping the
> states.  If a specific arch doesn't have proper probe_kernel_read()
> implementation, adding if (!NULL) test there could be a temporary
> workaround, but it should be clearly marked as such.

Sure, probe_kernel_read() takes care that no segfaults will happen.
Nevertheless, if we know that "pwq" might become NULL, why access pwq->wq at all?
  struct pool_workqueue *pwq = NULL;
  probe_kernel_read(&wq, &pwq>wq, sizeof(wq));

If you wouldn't have used probe_kernel_read() you would never code it 
like that. That's what I meant when I wrote "clean coding" (aka "similar
to what you would have done without probe_kernel_read()").

Helge
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