netdev - Re: [PATCH] rtnl: Simplify ASSERT

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Date:	Sat, 29 Sep 2007 11:18:18 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] rtnl: Simplify ASSERT_RTNL

Herbert Xu <herbert@...dor.apana.org.au> writes:

> Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> 
>> Currently we have the call path:
>> macvlan_open -> dev_unicast_add -> __dev_set_rx_mode ->
>>        __dev_set_promiscuity -> ASSERT_RTNL -> mutex_trylock
>> 
>> When mutex debugging is on taking a mutex complains if we are not
>> allowed to sleep.  At that point we have called netif_tx_lock_bh
>> so we are clearly not allowed to sleep.  Arguably this is not a
>> problem for mutex_trylock.
>
> Actually holding the TX lock here is a bug.  We're going to
> call down into the hardware with __dev_set_promiscuity, which
> may sleep (think USB NICs), so we definitely shouldn't be holding
> any spin locks.

Regardless of the correctness of where we have ASSERT_RTNL.
I think not actually taking the mutex on the assertion failure path
(just so we can release it), is still a good deal regardless.

For this particular call site clearly we need to look at what
is happening a little more.  The obvious thing would be to add
an explicit might_sleep if we are calling code that can sleep.

Eric
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