[1/2] carl9170: Fix wrong completion usage

Commit Message

Daniel Wagner Aug. 18, 2016, 1:12 p.m. UTC

From: Daniel Wagner <[e-mail]>

carl9170_usb_stop() is used from several places to flush and cleanup any
pending work. The normal pattern is to send a request and wait for the
irq handler to call complete(). The completion is not reinitialized
during normal operation and as the old comment indicates it is important
to keep calls to wait_for_completion_timeout() and complete() balanced.

Calling complete_all() brings this equilibirum out of balance and needs
to be fixed by a reinit_completion(). But that opens a small race
window. It is possible that the sequence of complete_all(),
reinit_completion() is faster than the wait_for_completion_timeout() can
do its work. The wake up is not lost but the done counter test is after
reinit_completion() has been executed. The only reason we don't see
carl9170_exec_cmd() hang forever is we use the timeout version of
wait_for_copletion().

Let's fix this by reinitializing the completion (that is just setting
done counter to 0) just before we send out an request. Now,
carl9170_usb_stop() can be sure a complete() call is enough to make
progess since there is only one waiter at max. This is a common pattern
also seen in various drivers which use completion.

Signed-off-by: Daniel Wagner <[e-mail]>
---
 drivers/net/wireless/ath/carl9170/usb.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

Comments

Kalle Valo Sept. 9, 2016, 12:16 p.m. UTC | #1

Daniel Wagner <[e-mail]> wrote:
> From: Daniel Wagner <[e-mail]>
> 
> carl9170_usb_stop() is used from several places to flush and cleanup any
> pending work. The normal pattern is to send a request and wait for the
> irq handler to call complete(). The completion is not reinitialized
> during normal operation and as the old comment indicates it is important
> to keep calls to wait_for_completion_timeout() and complete() balanced.
> 
> Calling complete_all() brings this equilibirum out of balance and needs
> to be fixed by a reinit_completion(). But that opens a small race
> window. It is possible that the sequence of complete_all(),
> reinit_completion() is faster than the wait_for_completion_timeout() can
> do its work. The wake up is not lost but the done counter test is after
> reinit_completion() has been executed. The only reason we don't see
> carl9170_exec_cmd() hang forever is we use the timeout version of
> wait_for_copletion().
> 
> Let's fix this by reinitializing the completion (that is just setting
> done counter to 0) just before we send out an request. Now,
> carl9170_usb_stop() can be sure a complete() call is enough to make
> progess since there is only one waiter at max. This is a common pattern
> also seen in various drivers which use completion.
> 
> Signed-off-by: Daniel Wagner <[e-mail]>

Thanks, 1 patch applied to ath-next branch of ath.git:

78a9e170388b carl9170: Fix wrong completion usage

Patch

diff --git a/drivers/net/wireless/ath/carl9170/usb.c b/drivers/net/wireless/ath/carl9170/usb.c
index 76842e6..99ab203 100644
--- a/drivers/net/wireless/ath/carl9170/usb.c
+++ b/drivers/net/wireless/ath/carl9170/usb.c
@@ -670,6 +670,7 @@  int carl9170_exec_cmd(struct ar9170 *ar, const enum carl9170_cmd_oids cmd,
 	ar->readlen = outlen;
 	spin_unlock_bh(&ar->cmd_lock);
 
+	reinit_completion(&ar->cmd_wait);
 	err = __carl9170_exec_cmd(ar, &ar->cmd, false);
 
 	if (!(cmd & CARL9170_CMD_ASYNC_FLAG)) {
@@ -778,10 +779,7 @@  void carl9170_usb_stop(struct ar9170 *ar)
 	spin_lock_bh(&ar->cmd_lock);
 	ar->readlen = 0;
 	spin_unlock_bh(&ar->cmd_lock);
-	complete_all(&ar->cmd_wait);
-
-	/* This is required to prevent an early completion on _start */
-	reinit_completion(&ar->cmd_wait);
+	complete(&ar->cmd_wait);
 
 	/*
 	 * Note: