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authorDavidlohr Bueso <davidlohr@hp.com>2014-01-27 17:07:10 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-01-27 21:02:40 -0800
commitffa571dafbaec0c74e374ce0ea7b4212b6cbc94c (patch)
tree21f19b4f4ddcf219c7b024c2cf7ffbac9d0a80b8 /ipc
parentdaf948c7d1a080041ae19aca07625efec670695a (diff)
downloadlinux-ffa571dafbaec0c74e374ce0ea7b4212b6cbc94c.tar.gz
ipc,msg: document barriers
Both expunge_all() and pipeline_send() rely on both a nil msg value and a full barrier to guarantee the correct ordering when waking up a task. While its counterpart at the receiving end is well documented for the lockless recv algorithm, we still need to document these specific smp_mb() calls. [akpm@linux-foundation.org: fix typo, per Mike] [akpm@linux-foundation.org: mroe tpyos] Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Rik van Riel <riel@redhat.com> Cc: Manfred Spraul <manfred@colorfullife.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'ipc')
-rw-r--r--ipc/msg.c19
1 files changed, 17 insertions, 2 deletions
diff --git a/ipc/msg.c b/ipc/msg.c
index 4377f4a4562..245db1140ad 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -253,8 +253,14 @@ static void expunge_all(struct msg_queue *msq, int res)
struct msg_receiver *msr, *t;
list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
- msr->r_msg = NULL;
+ 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_mb();
msr->r_msg = ERR_PTR(res);
}
@@ -638,15 +644,22 @@ 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);
- smp_mb();
+ smp_mb(); /* see barrier comment below */
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 end depends
+ * on it. See lockless receive part 1 and 2 in
+ * do_msgrcv().
+ */
smp_mb();
msr->r_msg = msg;
@@ -654,6 +667,7 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
}
}
}
+
return 0;
}
@@ -716,6 +730,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
goto out_unlock0;
}
+ /* enqueue the sender and prepare to block */
ss_add(msq, &s);
if (!ipc_rcu_getref(msq)) {