netdev - Re: [PATCH v3 02/22] netconsole: Introduce locking over the netpoll fields

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Message-ID: <AANLkTinVHrkLOMBkH3cauQndL03C_mnAJDJNYHa14ysZ@mail.gmail.com>
Date:	Tue, 14 Dec 2010 15:30:24 -0800
From:	Mike Waychison <mikew@...gle.com>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	simon.kagstrom@...insight.net, davem@...emloft.net,
	nhorman@...driver.com, Matt Mackall <mpm@...enic.com>,
	adurbin@...gle.com, linux-kernel@...r.kernel.org,
	chavey@...gle.com, Greg KH <greg@...ah.com>,
	netdev@...r.kernel.org,
	Américo Wang <xiyou.wangcong@...il.com>,
	akpm@...ux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 02/22] netconsole: Introduce locking over the netpoll fields

2010/12/14 Michał Mirosław <mirqus@...il.com>:
> 2010/12/14 Mike Waychison <mikew@...gle.com>:
>> The netconsole driver currently doesn't do any locking over its
>> configuration fields.  This can cause problems if we were to ever have
>> concurrent writing to fields while somebody is enabling the service.
>>
>> For simplicity, this patch extends targets_list_lock to cover all
>> configuration fields within the targets.  Macros are also added here to
>> wrap accessors so that we check whether the target has been enabled with
>> locking handled.
>>
>> Signed-off-by: Mike Waychison <mikew@...gle.com>
>> Acked-by: Matt Mackall <mpm@...enic.com>
>> ---
>>  drivers/net/netconsole.c |  114 ++++++++++++++++++++++++++--------------------
>>  1 files changed, 64 insertions(+), 50 deletions(-)
>>
>> diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
>> index c87a49e..6e16888 100644
>> --- a/drivers/net/netconsole.c
>> +++ b/drivers/net/netconsole.c
>> @@ -327,6 +327,7 @@ static ssize_t store_enabled(struct netconsole_target *nt,
>>                             const char *buf,
>>                             size_t count)
>>  {
>> +       unsigned long flags;
>>        int err;
>>        long enabled;
>>
>> @@ -335,6 +336,10 @@ static ssize_t store_enabled(struct netconsole_target *nt,
>>                return enabled;
>>
>>        if (enabled) {  /* 1 */
>> +               spin_lock_irqsave(&target_list_lock, flags);
>> +               if (nt->enabled)
>> +                       goto busy;
>> +               spin_unlock_irqrestore(&target_list_lock, flags);
>>
>
> This looks wrong. Unless there is another lock or mutex covering this
> function, at this point (after spin_unlock_irqrestore()) another
> thread might set nt->enabled = 1.
>

Agreed that this looks wrong :)

It is fixed in the next patch where a state machine is introduced to
replace the binary flag nt->enabled.  The code before this patch had
the a very similar problem in that a target could be enabled twice.
store_enabled() would call netpoll_setup() the second time without
checking to see if it was already enabled.
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