linux-kernel - RE: [PATCHv2] firmware: Correct handling of fw_state_wait

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Date:   Wed, 18 Jan 2017 06:33:56 +0000
From:   linux-kernel-dev <linux-kernel-dev@...khoff.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>
CC:     Chris Wilson <chris@...is-wilson.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Daniel Wagner <daniel.wagner@...-carit.de>,
        Ming Lei <ming.lei@...onical.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "oss-drivers@...ronome.com" <oss-drivers@...ronome.com>
Subject: RE: [PATCHv2] firmware: Correct handling of fw_state_wait_timeout()
 return value

>From: Jakub Kicinski [mailto:jakub.kicinski@...ronome.com]
>Sent: Dienstag, 17. Januar 2017 22:18
>
>On Tue, Jan 17, 2017 at 12:53 PM, Luis R. Rodriguez <mcgrof@...nel.org>
>wrote:
>> On Tue, Jan 17, 2017 at 10:04:20AM -0800, Jakub Kicinski wrote:
>>> On Tue, Jan 17, 2017 at 9:30 AM, Luis R. Rodriguez <mcgrof@...nel.org>
>wrote:
>>> > On Tue, Jan 17, 2017 at 08:30:37AM -0800, Jakub Kicinski wrote:
>>> >> Adding a NULL-check would just paper over the
>>> >> issue and can cause trouble down the line.
>>> >
>>> > We typically bail on errors and use similar code to bail out, and we
>>> > typically do these things. Here its no different. The *real* issue
>>> > is the fact that we have a waiting timeout which can fail race against
>>> > a user imposed error out on the sysfs interface. There is one catch:
>>> >
>>> > We already lock with the big fw_lock and use this to be able to check
>>> > for the status of the fw, so once aborted we technically should not have
>>> > to abort again. A proper way to address then this would have been to
>check
>>> > for the status of the fw prior to aborting again given we also lock on the
>>> > big fw_lock. A problem with this though is the status is part of the buf
>>> > which is set to NULL after we are done aborting.
>>>
>>> Yes, I've seen that too :\  This race seems to have been there prior
>>> to 4.9, though.  I guess we could fix both issues with the NULL-check
>>> although I would prefer if we had both patches.
>>>
>>> FWIW I think the NULL-check could be put in the existing conditional:
>>>
>>>          * There is a small window in which user can write to 'loading'
>>>          * between loading done and disappearance of 'loading'
>>>          */
>>> -       if (fw_state_is_done(&buf->fw_st))
>>> +       if (!buf || fw_state_is_done(&buf->fw_st))
>>>                 return;
>>>
>>>         list_del_init(&buf->pending_list);
>>>
>>> Note that the comment above seems to be mentioning the race we're
>>> trying to solve.
>>
>> Right, I think another approach is to *enable* the state of the buf
>> to be used to avoid further use on the sysfs iterface instead. Fortunately
>> other sysfs interfaces already use fw_state_is_done() to bail out,
>> so all that would be needed I think would be:
>>
>> diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
>> index b9ac348e8d33..30ccf7aea3ca 100644
>> --- a/drivers/base/firmware_class.c
>> +++ b/drivers/base/firmware_class.c
>> @@ -558,9 +558,6 @@ static void fw_load_abort(struct firmware_priv
>*fw_priv)
>>         struct firmware_buf *buf = fw_priv->buf;
>>
>>         __fw_load_abort(buf);
>> -
>> -       /* avoid user action after loading abort */
>> -       fw_priv->buf = NULL;
>>  }
>>
>>  static LIST_HEAD(pending_fw_head);
>> @@ -713,7 +710,7 @@ static ssize_t firmware_loading_store(struct device
>*dev,
>>
>>         mutex_lock(&fw_lock);
>>         fw_buf = fw_priv->buf;
>> -       if (!fw_buf)
>> +       if (!fw_buf || fw_state_is_aborted(&fw_buf->fw_st))
>>                 goto out;
>>
>>         switch (loading) {
>
>IMHO this one is nice!  I think you can even drop the !fw_buf check in
>this case because AFAICS the only case where fw_buf is set to NULL is
>in the abort function.
>
I can confirm, that patch looks nice and is working for my setup, even without the !fw_buf. 
Feel free to grab everything you need from my commit log, if it helps.
Unfortunately there is a crazy spam filter between us, so you can't rely on me.