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| author | Davidlohr Bueso <davidlohr@hp.com> | 2014-01-27 17:07:10 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2014-01-27 21:02:40 -0800 |
| commit | ffa571dafbaec0c74e374ce0ea7b4212b6cbc94c (patch) | |
| tree | 21f19b4f4ddcf219c7b024c2cf7ffbac9d0a80b8 /ipc | |
| parent | daf948c7d1a080041ae19aca07625efec670695a (diff) | |
| download | linux-ffa571dafbaec0c74e374ce0ea7b4212b6cbc94c.tar.bz2 | |
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.c | 19 |
1 files changed, 17 insertions, 2 deletions
diff --git a/ipc/msg.c b/ipc/msg.c index 4377f4a45625..245db1140ad6 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)) { |