diff options
Diffstat (limited to 'ipc/msg.c')
| -rw-r--r-- | ipc/msg.c | 48 |
1 files changed, 38 insertions, 10 deletions
@@ -196,7 +196,7 @@ static void expunge_all(struct msg_queue *msq, int res) * or dealing with -EAGAIN cases. See lockless receive part 1 * and 2 in do_msgrcv(). */ - smp_mb(); + smp_wmb(); /* barrier (B) */ msr->r_msg = ERR_PTR(res); } } @@ -580,7 +580,8 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) /* initialize pipelined send ordering */ msr->r_msg = NULL; wake_up_process(msr->r_tsk); - smp_mb(); /* see barrier comment below */ + /* barrier (B) see barrier comment below */ + smp_wmb(); msr->r_msg = ERR_PTR(-E2BIG); } else { msr->r_msg = NULL; @@ -589,11 +590,12 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) wake_up_process(msr->r_tsk); /* * Ensure that the wakeup is visible before - * setting r_msg, as the receiving end depends - * on it. See lockless receive part 1 and 2 in - * do_msgrcv(). + * 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_mb(); + smp_wmb(); msr->r_msg = msg; return 1; @@ -932,12 +934,38 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl /* 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. + * 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; + * + * 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. */ - msg = (struct msg_msg *)msr_d.r_msg; - while (msg == NULL) { - cpu_relax(); + 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: |
