RDMA/cxgb4: Use completion objects for event blocking

There exists a race condition when using wait_queue_head_t objects
that are declared on the stack.  This was being done in a few places
where we are sending work requests to the FW and awaiting replies, but
we don't have an endpoint structure with an embedded c4iw_wr_wait
struct.  So the code was allocating it locally on the stack.  Bad
design.  The race is:

  1) thread on cpuX declares the wait_queue_head_t on the stack, then
     posts a firmware WR with that wait object ptr as the cookie to be
     returned in the WR reply.  This thread will proceed to block in
     wait_event_timeout() but before it does:

  2) An interrupt runs on cpuY with the WR reply.  fw6_msg() handles
     this and calls c4iw_wake_up().  c4iw_wake_up() sets the condition
     variable in the c4iw_wr_wait object to TRUE and will call
     wake_up(), but before it calls wake_up():

  3) The thread on cpuX calls c4iw_wait_for_reply(), which calls
     wait_event_timeout().  The wait_event_timeout() macro checks the
     condition variable and returns immediately since it is TRUE.  So
     this thread never blocks/sleeps. The function then returns
     effectively deallocating the c4iw_wr_wait object that was on the
     stack.

  4) So at this point cpuY has a pointer to the c4iw_wr_wait object
     that is no longer valid.  Further its pointing to a stack frame
     that might now be in use by some other context/thread.  So cpuY
     continues execution and calls wake_up() on a ptr to a wait object
     that as been effectively deallocated.

This race, when it hits, can cause a crash in wake_up(), which I've
seen under heavy stress. It can also corrupt the referenced stack
which can cause any number of failures.

The fix:

Use struct completion, which supports on-stack declarations.
Completions use a spinlock around setting the condition to true and
the wake up so that steps 2 and 4 above are atomic and step 3 can
never happen in-between.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>

drivers/infiniband/hw/cxgb4/iw_cxgb4.h

@@ -35,7 +35,7 @@
 #include <linux/list.h>
 #include <linux/spinlock.h>
 #include <linux/idr.h>
-#include <linux/workqueue.h>
+#include <linux/completion.h>
 #include <linux/netdevice.h>
 #include <linux/sched.h>
 #include <linux/pci.h>
@@ -131,28 +131,21 @@ static inline int c4iw_num_stags(struct c4iw_rdev *rdev)
 
 #define C4IW_WR_TO (10*HZ)
 
-enum {
-	REPLY_READY = 0,
-};
-
 struct c4iw_wr_wait {
-	wait_queue_head_t wait;
-	unsigned long status;
+	struct completion completion;
 	int ret;
 };
 
 static inline void c4iw_init_wr_wait(struct c4iw_wr_wait *wr_waitp)
 {
 	wr_waitp->ret = 0;
-	wr_waitp->status = 0;
-	init_waitqueue_head(&wr_waitp->wait);
+	init_completion(&wr_waitp->completion);
 }
 
 static inline void c4iw_wake_up(struct c4iw_wr_wait *wr_waitp, int ret)
 {
 	wr_waitp->ret = ret;
-	set_bit(REPLY_READY, &wr_waitp->status);
-	wake_up(&wr_waitp->wait);
+	complete(&wr_waitp->completion);
 }
 
 static inline int c4iw_wait_for_reply(struct c4iw_rdev *rdev,
@@ -164,8 +157,7 @@ static inline int c4iw_wait_for_reply(struct c4iw_rdev *rdev,
 	int ret;
 
 	do {
-		ret = wait_event_timeout(wr_waitp->wait,
-			test_and_clear_bit(REPLY_READY, &wr_waitp->status), to);
+		ret = wait_for_completion_timeout(&wr_waitp->completion, to);
 		if (!ret) {
 			printk(KERN_ERR MOD "%s - Device %s not responding - "
 			       "tid %u qpid %u\n", func,