X-Git-Url: http://git.cascardo.info/?a=blobdiff_plain;f=ipc%2Fmsg.c;h=e12307d0c920ccd9e50168ed890c5b6057206a52;hb=689f891c980949d3eb64f61651db53cb347e0a13;hp=c6521c205cb403a81cc2bf4e6969b2a5d54620c3;hpb=e8f0f8aa4eb356bd729f15be8827db2558ea1ae6;p=cascardo%2Flinux.git

diff --git a/ipc/msg.c b/ipc/msg.c
index c6521c205cb4..e12307d0c920 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -51,19 +51,14 @@ struct msg_receiver {
 	long			r_msgtype;
 	long			r_maxsize;
 
-	/*
-	 * Mark r_msg volatile so that the compiler
-	 * does not try to get smart and optimize
-	 * it. We rely on this for the lockless
-	 * receive algorithm.
-	 */
-	struct msg_msg		*volatile r_msg;
+	struct msg_msg		*r_msg;
 };
 
 /* one msg_sender for each sleeping sender */
 struct msg_sender {
 	struct list_head	list;
 	struct task_struct	*tsk;
+	size_t                  msgsz;
 };
 
 #define SEARCH_ANY		1
@@ -159,45 +154,72 @@ static int newque(struct ipc_namespace *ns, struct ipc_params *params)
 	return msq->q_perm.id;
 }
 
-static inline void ss_add(struct msg_queue *msq, struct msg_sender *mss)
+static inline bool msg_fits_inqueue(struct msg_queue *msq, size_t msgsz)
+{
+	return msgsz + msq->q_cbytes <= msq->q_qbytes &&
+		1 + msq->q_qnum <= msq->q_qbytes;
+}
+
+static inline void ss_add(struct msg_queue *msq,
+			  struct msg_sender *mss, size_t msgsz)
 {
 	mss->tsk = current;
+	mss->msgsz = msgsz;
 	__set_current_state(TASK_INTERRUPTIBLE);
 	list_add_tail(&mss->list, &msq->q_senders);
 }
 
 static inline void ss_del(struct msg_sender *mss)
 {
-	if (mss->list.next != NULL)
+	if (mss->list.next)
 		list_del(&mss->list);
 }
 
-static void ss_wakeup(struct list_head *h, int kill)
+static void ss_wakeup(struct msg_queue *msq,
+		      struct wake_q_head *wake_q, bool kill)
 {
 	struct msg_sender *mss, *t;
+	struct task_struct *stop_tsk = NULL;
+	struct list_head *h = &msq->q_senders;
 
 	list_for_each_entry_safe(mss, t, h, list) {
 		if (kill)
 			mss->list.next = NULL;
-		wake_up_process(mss->tsk);
+
+		/*
+		 * Stop at the first task we don't wakeup,
+		 * we've already iterated the original
+		 * sender queue.
+		 */
+		else if (stop_tsk == mss->tsk)
+			break;
+		/*
+		 * We are not in an EIDRM scenario here, therefore
+		 * verify that we really need to wakeup the task.
+		 * To maintain current semantics and wakeup order,
+		 * move the sender to the tail on behalf of the
+		 * blocked task.
+		 */
+		else if (!msg_fits_inqueue(msq, mss->msgsz)) {
+			if (!stop_tsk)
+				stop_tsk = mss->tsk;
+
+			list_move_tail(&mss->list, &msq->q_senders);
+			continue;
+		}
+
+		wake_q_add(wake_q, mss->tsk);
 	}
 }
 
-static void expunge_all(struct msg_queue *msq, int res)
+static void expunge_all(struct msg_queue *msq, int res,
+			struct wake_q_head *wake_q)
 {
 	struct msg_receiver *msr, *t;
 
 	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
-		msr->r_msg = NULL; /* initialize expunge ordering */
-		wake_up_process(msr->r_tsk);
-		/*
-		 * Ensure that the wakeup is visible before setting r_msg as
-		 * the receiving end depends on it: either spinning on a nil,
-		 * or dealing with -EAGAIN cases. See lockless receive part 1
-		 * and 2 in do_msgrcv().
-		 */
-		smp_wmb(); /* barrier (B) */
-		msr->r_msg = ERR_PTR(res);
+		wake_q_add(wake_q, msr->r_tsk);
+		WRITE_ONCE(msr->r_msg, ERR_PTR(res));
 	}
 }
 
@@ -213,11 +235,13 @@ static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
 {
 	struct msg_msg *msg, *t;
 	struct msg_queue *msq = container_of(ipcp, struct msg_queue, q_perm);
+	WAKE_Q(wake_q);
 
-	expunge_all(msq, -EIDRM);
-	ss_wakeup(&msq->q_senders, 1);
+	expunge_all(msq, -EIDRM, &wake_q);
+	ss_wakeup(msq, &wake_q, true);
 	msg_rmid(ns, msq);
 	ipc_unlock_object(&msq->q_perm);
+	wake_up_q(&wake_q);
 	rcu_read_unlock();
 
 	list_for_each_entry_safe(msg, t, &msq->q_messages, m_list) {
@@ -372,6 +396,9 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 		freeque(ns, ipcp);
 		goto out_up;
 	case IPC_SET:
+	{
+		WAKE_Q(wake_q);
+
 		if (msqid64.msg_qbytes > ns->msg_ctlmnb &&
 		    !capable(CAP_SYS_RESOURCE)) {
 			err = -EPERM;
@@ -386,15 +413,21 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 		msq->q_qbytes = msqid64.msg_qbytes;
 
 		msq->q_ctime = get_seconds();
-		/* sleeping receivers might be excluded by
+		/*
+		 * Sleeping receivers might be excluded by
 		 * stricter permissions.
 		 */
-		expunge_all(msq, -EAGAIN);
-		/* sleeping senders might be able to send
+		expunge_all(msq, -EAGAIN, &wake_q);
+		/*
+		 * Sleeping senders might be able to send
 		 * due to a larger queue size.
 		 */
-		ss_wakeup(&msq->q_senders, 0);
-		break;
+		ss_wakeup(msq, &wake_q, false);
+		ipc_unlock_object(&msq->q_perm);
+		wake_up_q(&wake_q);
+
+		goto out_unlock1;
+	}
 	default:
 		err = -EINVAL;
 		goto out_unlock1;
@@ -566,7 +599,8 @@ static int testmsg(struct msg_msg *msg, long type, int mode)
 	return 0;
 }
 
-static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
+static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg,
+				 struct wake_q_head *wake_q)
 {
 	struct msg_receiver *msr, *t;
 
@@ -577,27 +611,14 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
 
 			list_del(&msr->r_list);
 			if (msr->r_maxsize < msg->m_ts) {
-				/* initialize pipelined send ordering */
-				msr->r_msg = NULL;
-				wake_up_process(msr->r_tsk);
-				/* barrier (B) see barrier comment below */
-				smp_wmb();
-				msr->r_msg = ERR_PTR(-E2BIG);
+				wake_q_add(wake_q, msr->r_tsk);
+				WRITE_ONCE(msr->r_msg, ERR_PTR(-E2BIG));
 			} else {
-				msr->r_msg = NULL;
 				msq->q_lrpid = task_pid_vnr(msr->r_tsk);
 				msq->q_rtime = get_seconds();
-				wake_up_process(msr->r_tsk);
-				/*
-				 * Ensure that the wakeup is visible before
-				 * setting r_msg, as the receiving can otherwise
-				 * exit - once r_msg is set, the receiver can
-				 * continue. See lockless receive part 1 and 2
-				 * in do_msgrcv(). Barrier (B).
-				 */
-				smp_wmb();
-				msr->r_msg = msg;
 
+				wake_q_add(wake_q, msr->r_tsk);
+				WRITE_ONCE(msr->r_msg, msg);
 				return 1;
 			}
 		}
@@ -613,6 +634,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 	struct msg_msg *msg;
 	int err;
 	struct ipc_namespace *ns;
+	WAKE_Q(wake_q);
 
 	ns = current->nsproxy->ipc_ns;
 
@@ -654,10 +676,8 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 		if (err)
 			goto out_unlock0;
 
-		if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
-				1 + msq->q_qnum <= msq->q_qbytes) {
+		if (msg_fits_inqueue(msq, msgsz))
 			break;
-		}
 
 		/* queue full, wait: */
 		if (msgflg & IPC_NOWAIT) {
@@ -666,7 +686,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 		}
 
 		/* enqueue the sender and prepare to block */
-		ss_add(msq, &s);
+		ss_add(msq, &s, msgsz);
 
 		if (!ipc_rcu_getref(msq)) {
 			err = -EIDRM;
@@ -686,7 +706,6 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 			err = -EIDRM;
 			goto out_unlock0;
 		}
-
 		ss_del(&s);
 
 		if (signal_pending(current)) {
@@ -695,10 +714,11 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 		}
 
 	}
+
 	msq->q_lspid = task_tgid_vnr(current);
 	msq->q_stime = get_seconds();
 
-	if (!pipelined_send(msq, msg)) {
+	if (!pipelined_send(msq, msg, &wake_q)) {
 		/* no one is waiting for this message, enqueue it */
 		list_add_tail(&msg->m_list, &msq->q_messages);
 		msq->q_cbytes += msgsz;
@@ -712,6 +732,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 
 out_unlock0:
 	ipc_unlock_object(&msq->q_perm);
+	wake_up_q(&wake_q);
 out_unlock1:
 	rcu_read_unlock();
 	if (msg != NULL)
@@ -829,6 +850,7 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 	struct msg_queue *msq;
 	struct ipc_namespace *ns;
 	struct msg_msg *msg, *copy = NULL;
+	WAKE_Q(wake_q);
 
 	ns = current->nsproxy->ipc_ns;
 
@@ -893,7 +915,7 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 			msq->q_cbytes -= msg->m_ts;
 			atomic_sub(msg->m_ts, &ns->msg_bytes);
 			atomic_dec(&ns->msg_hdrs);
-			ss_wakeup(&msq->q_senders, 0);
+			ss_wakeup(msq, &wake_q, false);
 
 			goto out_unlock0;
 		}
@@ -919,71 +941,38 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 		rcu_read_unlock();
 		schedule();
 
-		/* Lockless receive, part 1:
-		 * Disable preemption.  We don't hold a reference to the queue
-		 * and getting a reference would defeat the idea of a lockless
-		 * operation, thus the code relies on rcu to guarantee the
-		 * existence of msq:
+		/*
+		 * Lockless receive, part 1:
+		 * We don't hold a reference to the queue and getting a
+		 * reference would defeat the idea of a lockless operation,
+		 * thus the code relies on rcu to guarantee the existence of
+		 * msq:
 		 * Prior to destruction, expunge_all(-EIRDM) changes r_msg.
 		 * Thus if r_msg is -EAGAIN, then the queue not yet destroyed.
-		 * rcu_read_lock() prevents preemption between reading r_msg
-		 * and acquiring the q_perm.lock in ipc_lock_object().
 		 */
 		rcu_read_lock();
 
-		/* Lockless receive, part 2:
-		 * Wait until pipelined_send or expunge_all are outside of
-		 * wake_up_process(). There is a race with exit(), see
-		 * ipc/mqueue.c for the details. The correct serialization
-		 * ensures that a receiver cannot continue without the wakeup
-		 * being visibible _before_ setting r_msg:
-		 *
-		 * CPU 0                             CPU 1
-		 * <loop receiver>
-		 *   smp_rmb(); (A) <-- pair -.      <waker thread>
-		 *   <load ->r_msg>           |        msr->r_msg = NULL;
-		 *                            |        wake_up_process();
-		 * <continue>                 `------> smp_wmb(); (B)
-		 *                                     msr->r_msg = msg;
+		/*
+		 * Lockless receive, part 2:
+		 * The work in pipelined_send() and expunge_all():
+		 * - Set pointer to message
+		 * - Queue the receiver task for later wakeup
+		 * - Wake up the process after the lock is dropped.
 		 *
-		 * Where (A) orders the message value read and where (B) orders
-		 * the write to the r_msg -- done in both pipelined_send and
-		 * expunge_all.
-		 */
-		for (;;) {
-			/*
-			 * Pairs with writer barrier in pipelined_send
-			 * or expunge_all.
-			 */
-			smp_rmb(); /* barrier (A) */
-			msg = (struct msg_msg *)msr_d.r_msg;
-			if (msg)
-				break;
-
-			/*
-			 * The cpu_relax() call is a compiler barrier
-			 * which forces everything in this loop to be
-			 * re-loaded.
-			 */
-			cpu_relax();
-		}
-
-		/* Lockless receive, part 3:
-		 * If there is a message or an error then accept it without
-		 * locking.
+		 * Should the process wake up before this wakeup (due to a
+		 * signal) it will either see the message and continue ...
 		 */
+		msg = READ_ONCE(msr_d.r_msg);
 		if (msg != ERR_PTR(-EAGAIN))
 			goto out_unlock1;
 
-		/* Lockless receive, part 3:
-		 * Acquire the queue spinlock.
-		 */
+		 /*
+		  * ... or see -EAGAIN, acquire the lock to check the message
+		  * again.
+		  */
 		ipc_lock_object(&msq->q_perm);
 
-		/* Lockless receive, part 4:
-		 * Repeat test after acquiring the spinlock.
-		 */
-		msg = (struct msg_msg *)msr_d.r_msg;
+		msg = msr_d.r_msg;
 		if (msg != ERR_PTR(-EAGAIN))
 			goto out_unlock0;
 
@@ -998,6 +987,7 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 
 out_unlock0:
 	ipc_unlock_object(&msq->q_perm);
+	wake_up_q(&wake_q);
 out_unlock1:
 	rcu_read_unlock();
 	if (IS_ERR(msg)) {