[PATCH v2] ipc/mqueue: remove STATE_PENDING

[Sebastian Andrzej Siewior]

This patch moves the wakeup_process() invocation so it is not done under
the info->lock. With this change, the waiter is woken up once it is
"ready" which means its state is STATE_READY and it does not need to loop
on SMP if it is still in STATE_PENDING.
In the timeout case we still need to grab the info->lock to verify the state.

This change should also avoid the introduction of preempt_disable() in
-RT which avoids a busy-loop which pools for the STATE_PENDING -> STATE_READY
change if the waiter has a higher priority compared to the waker.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
v1âv2: update / correct the comment of how the algorithm works now.

* Manfred Spraul | 2015-04-07 19:48:01 [+0200]:
>No. With your change, ipc/sem.c and ipc/msg.c use different algorithms.
>Please update the comment and describe the new approach:
>
>Current approach:
>- set pointer to message
>- STATE_PENDING
>- wake_up_process()
>- STATE_READY
>    (now the receiver can continue)
>
>New approach:
>- set pointer to message
>- get_task_struct
>- STATE_READY
>    (now the receiver can continue, e.g. woken up due to an unrelated
>SIGKILL)
>- wake_up_process()
>- put_task_struct()

Updated. Does it meet your expectation?

>
>>+		if (r_sender) {
>>+			wake_up_process(r_sender);
>>+			put_task_struct(r_sender);
>>+		}
>>  		ret = 0;
>Could you double-check that it is safe to call wake_up_process on a
>killed and reaped thread, only with a get_task_struct reference?
tglx answered that part.

>And: please test it, too. (patch the kernel so that you can trigger
>this case).

Why patch? Isn't this triggered if you have a reader waiting and you
send a message?

 ipc/mqueue.c | 51 ++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 34 insertions(+), 17 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 7635a1cf99f3..bfdc2219a470 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -47,7 +47,6 @@
 #define RECV		1
 
 #define STATE_NONE	0
-#define STATE_PENDING	1
 #define STATE_READY	2
 
 struct posix_msg_tree_node {
@@ -577,9 +576,6 @@ static int wq_sleep(struct mqueue_inode_info *info, int sr,
 		time = schedule_hrtimeout_range_clock(timeout, 0,
 			HRTIMER_MODE_ABS, CLOCK_REALTIME);
 
-		while (ewp->state == STATE_PENDING)
-			cpu_relax();
-
 		if (ewp->state == STATE_READY) {
 			retval = 0;
 			goto out;
@@ -909,9 +905,14 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
  * bypasses the message array and directly hands the message over to the
  * receiver.
  * The receiver accepts the message and returns without grabbing the queue
- * spinlock. Therefore an intermediate STATE_PENDING state and memory barriers
- * are necessary. The same algorithm is used for sysv semaphores, see
- * ipc/sem.c for more details.
+ * spinlock. The used algorithm is different from sysv semaphores (ipc/sem.c):
+ * - set pointer to message
+ * - hold a reference of the task to be woken up
+ * - update its state (to STATE_READY). Now the receiver can continue.
+ * - wake up the process after the lock is dropped. Should the process wake up
+ *   before this wakeup (due to a timeout or a signal) it will either see
+ *   STATE_READY and continue or acquire the lock to check the sate again.
+ * - put the reference to task to be woken up.
  *
  * The same algorithm is used for senders.
  */
@@ -919,36 +920,41 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
 /* pipelined_send() - send a message directly to the task waiting in
  * sys_mq_timedreceive() (without inserting message into a queue).
  */
-static inline void pipelined_send(struct mqueue_inode_info *info,
+static struct task_struct *pipelined_send(struct mqueue_inode_info *info,
 				  struct msg_msg *message,
 				  struct ext_wait_queue *receiver)
 {
+	struct task_struct *r_task;
+
 	receiver->msg = message;
 	list_del(&receiver->list);
-	receiver->state = STATE_PENDING;
-	wake_up_process(receiver->task);
+	r_task = receiver->task;
+	get_task_struct(r_task);
 	smp_wmb();
 	receiver->state = STATE_READY;
+	return r_task;
 }
 
 /* pipelined_receive() - if there is task waiting in sys_mq_timedsend()
  * gets its message and put to the queue (we have one free place for sure). */
-static inline void pipelined_receive(struct mqueue_inode_info *info)
+static struct task_struct *pipelined_receive(struct mqueue_inode_info *info)
 {
+	struct task_struct *r_sender;
 	struct ext_wait_queue *sender = wq_get_first_waiter(info, SEND);
 
 	if (!sender) {
 		/* for poll */
 		wake_up_interruptible(&info->wait_q);
-		return;
+		return NULL;
 	}
 	if (msg_insert(sender->msg, info))
-		return;
+		return NULL;
 	list_del(&sender->list);
-	sender->state = STATE_PENDING;
-	wake_up_process(sender->task);
+	r_sender = sender->task;
+	get_task_struct(r_sender);
 	smp_wmb();
 	sender->state = STATE_READY;
+	return r_sender;
 }
 
 SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
@@ -961,6 +967,7 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
 	struct ext_wait_queue *receiver;
 	struct msg_msg *msg_ptr;
 	struct mqueue_inode_info *info;
+	struct task_struct *r_task = NULL;
 	ktime_t expires, *timeout = NULL;
 	struct timespec ts;
 	struct posix_msg_tree_node *new_leaf = NULL;
@@ -1049,7 +1056,7 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
 	} else {
 		receiver = wq_get_first_waiter(info, RECV);
 		if (receiver) {
-			pipelined_send(info, msg_ptr, receiver);
+			r_task = pipelined_send(info, msg_ptr, receiver);
 		} else {
 			/* adds message to the queue */
 			ret = msg_insert(msg_ptr, info);
@@ -1062,6 +1069,10 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr,
 	}
 out_unlock:
 	spin_unlock(&info->lock);
+	if (r_task) {
+		wake_up_process(r_task);
+		put_task_struct(r_task);
+	}
 out_free:
 	if (ret)
 		free_msg(msg_ptr);
@@ -1149,14 +1160,20 @@ SYSCALL_DEFINE5(mq_timedreceive, mqd_t, mqdes, char __user *, u_msg_ptr,
 			msg_ptr = wait.msg;
 		}
 	} else {
+		struct task_struct *r_sender;
+
 		msg_ptr = msg_get(info);
 
 		inode->i_atime = inode->i_mtime = inode->i_ctime =
 				CURRENT_TIME;
 
 		/* There is now free space in queue. */
-		pipelined_receive(info);
+		r_sender = pipelined_receive(info);
 		spin_unlock(&info->lock);
+		if (r_sender) {
+			wake_up_process(r_sender);
+			put_task_struct(r_sender);
+		}
 		ret = 0;
 	}
 	if (ret == 0) {
-- 
2.1.4

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