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ipc,msg: document barriers

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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>
This commit is contained in:
Davidlohr Bueso 2014-01-27 17:07:10 -08:00 committed by Linus Torvalds
parent daf948c7d1
commit ffa571dafb
1 changed files with 17 additions and 2 deletions
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19
ipc/msg.c
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@ -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)) {