From owner-freebsd-current@freebsd.org  Thu Feb 11 10:17:38 2016
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Subject: Re: Open Sound System - OSS "soundon" command causes KERNEL PANIC
 FreeBSD-11
To: Konstantin Belousov <kostikbel@gmail.com>
References: <56BB0BDB.2090203@selasky.org>
 <872763758.2494245.1455156152295.JavaMail.yahoo@mail.yahoo.com>
 <56BC3041.7050203@selasky.org> <20160211101418.GO91220@kib.kiev.ua>
Cc: Greg Quinlan <gwq_uk@yahoo.com>,
 "freebsd-current@freebsd.org" <freebsd-current@freebsd.org>
From: Hans Petter Selasky <hps@selasky.org>
Message-ID: <56BC604C.4060206@selasky.org>
Date: Thu, 11 Feb 2016 11:19:56 +0100
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On 02/11/16 11:14, Konstantin Belousov wrote:
> On Thu, Feb 11, 2016 at 07:54:57AM +0100, Hans Petter Selasky wrote:
>> On 02/11/16 03:02, Greg Quinlan wrote:
>>> Hi HPS,
>>> Note: Does not happen on FreeBSD 10.1-Stable!
>>>
>>
>> Yes, that's because WITNESS is off in 10.x by default.
>>
>> Does the attached patch solve your problem?
 >
> No, the patch below does not solve the issue, it only papers over it.
> I object against committing this change.

For cases where the returned pointer is not deferred, but only checked 
for a module's presence in the kernel you don't need a lock to protect 
anything. Maybe make a separate API for this?

>
> Issue is that, if called unlocked, the result from module_lookupbyname()
> could become invalid right after receiving.  It is the duty of the caller
> of the function to ensure that the result is still valid, and the only
> way to achieve it is to own the lock around the whole code region which
> calls the function and utilizes its result.
>
> A bug is in the OSS code.

Yes, so why not factor out the solution? Maybe more port KLD's will trip 
over this?

--HPS