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carl9170: explain why sta cannot be NULL for ampdus

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Dan Carpenter reported that smatch detected a potential
problem with the code [1]:
 drivers/net/wireless/ath/carl9170/tx.c:1488 carl9170_op_tx()
  error: we previously assumed 'sta' could be null (see line 1482)

 drivers/net/wireless/ath/carl9170/tx.c
  1482  if (sta) {
           ^^^^^ New check.
  [...]
  1485  }
  1487  if (info->flags & IEEE80211_TX_CTL_AMPDU) {
  1488    run = carl9170_tx_ampdu_queue(ar, sta, skb);
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   Old dereference of "sta" inside the call to carl9170_tx_ampdu_queue().

A range of solutions have been discussed in [2] and
we agreed on the following: "
> we might as well add a comment to carl9170_tx_ampdu_queue
> and explain the situation [in a way that's obvious to a
> human reader]. This way we can save the "if"... which is
> a small win since carl9170_op_tx is sort of a hot-path.
Putting a comment there is fine. Without the comment
it's easy for a human reader to get confused why the
check is there. So long as humans can read the code,
that's all that matters."

[1] <http://www.spinics.net/lists/linux-wireless/msg94526.html>
[2] <http://www.spinics.net/lists/linux-kernel-janitors/msg14953.html>

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
This commit is contained in:
Christian Lamparter 2012-12-03 18:48:05 +01:00 committed by John W. Linville
parent f241b244dd
commit 1981e881fd
1 changed files with 7 additions and 0 deletions
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7
drivers/net/wireless/ath/carl9170/tx.c
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@ -1485,6 +1485,13 @@ void carl9170_op_tx(struct ieee80211_hw *hw,
}
if (info->flags & IEEE80211_TX_CTL_AMPDU) {
/* to static code analyzers and reviewers:
* mac80211 guarantees that a valid "sta"
* reference is present, if a frame is to
* be part of an ampdu. Hence any extra
* sta == NULL checks are redundant in this
* special case.
*/
run = carl9170_tx_ampdu_queue(ar, sta, skb);
if (run)
carl9170_tx_ampdu(ar);