The patch titled
Subject: ipc/msg.c: update and document memory barriers
has been added to the -mm tree. Its filename is
ipc-msgc-update-and-document-memory-barriers.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/ipc-msgc-update-and-document-memory-barriers.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/ipc-msgc-update-and-document-memory-barriers.patch
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------------------------------------------------------
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Subject: ipc/msg.c: update and document memory barriers
Transfer findings from ipc/mqueue.c:
- A control barrier was missing for the lockless receive case So in
theory, not yet initialized data may have been copied to user space -
obviously only for architectures where control barriers are not NOP.
- use smp_store_release(). In theory, the refount may have been
decreased to 0 already when wake_q_add() tries to get a reference.
Link: http://lkml.kernel.org/r/20191020123305.14715-5-manfred@xxxxxxxxxxxxxxxx
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: <1vier1@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
ipc/msg.c | 43 ++++++++++++++++++++++++++++++++++++-------
1 file changed, 36 insertions(+), 7 deletions(-)
--- a/ipc/msg.c~ipc-msgc-update-and-document-memory-barriers
+++ a/ipc/msg.c
@@ -61,6 +61,16 @@ struct msg_queue {
struct list_head q_senders;
} __randomize_layout;
+/*
+ * MSG_BARRIER Locking:
+ *
+ * Similar to the optimization used in ipc/mqueue.c, one syscall return path
+ * does not acquire any locks when it sees that a message exists in
+ * msg_receiver.r_msg. Therefore r_msg is set using smp_store_release()
+ * and accessed using READ_ONCE()+smp_acquire__after_ctrl_dep(). In addition,
+ * wake_q_add_safe() is used. See ipc/mqueue.c for more details
+ */
+
/* one msg_receiver structure for each sleeping receiver */
struct msg_receiver {
struct list_head r_list;
@@ -184,6 +194,10 @@ static inline void ss_add(struct msg_que
{
mss->tsk = current;
mss->msgsz = msgsz;
+ /*
+ * No memory barrier required: we did ipc_lock_object(),
+ * and the waker obtains that lock before calling wake_q_add().
+ */
__set_current_state(TASK_INTERRUPTIBLE);
list_add_tail(&mss->list, &msq->q_senders);
}
@@ -237,8 +251,11 @@ static void expunge_all(struct msg_queue
struct msg_receiver *msr, *t;
list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
- wake_q_add(wake_q, msr->r_tsk);
- WRITE_ONCE(msr->r_msg, ERR_PTR(res));
+ get_task_struct(msr->r_tsk);
+
+ /* see MSG_BARRIER for purpose/pairing */
+ smp_store_release(&msr->r_msg, ERR_PTR(res));
+ wake_q_add_safe(wake_q, msr->r_tsk);
}
}
@@ -798,13 +815,17 @@ static inline int pipelined_send(struct
list_del(&msr->r_list);
if (msr->r_maxsize < msg->m_ts) {
wake_q_add(wake_q, msr->r_tsk);
- WRITE_ONCE(msr->r_msg, ERR_PTR(-E2BIG));
+
+ /* See expunge_all regarding memory barrier */
+ smp_store_release(&msr->r_msg, ERR_PTR(-E2BIG));
} else {
ipc_update_pid(&msq->q_lrpid, task_pid(msr->r_tsk));
msq->q_rtime = ktime_get_real_seconds();
wake_q_add(wake_q, msr->r_tsk);
- WRITE_ONCE(msr->r_msg, msg);
+
+ /* See expunge_all regarding memory barrier */
+ smp_store_release(&msr->r_msg, msg);
return 1;
}
}
@@ -1154,7 +1175,11 @@ static long do_msgrcv(int msqid, void __
msr_d.r_maxsize = INT_MAX;
else
msr_d.r_maxsize = bufsz;
- msr_d.r_msg = ERR_PTR(-EAGAIN);
+
+ /* memory barrier not require due to ipc_lock_object() */
+ WRITE_ONCE(msr_d.r_msg, ERR_PTR(-EAGAIN));
+
+ /* memory barrier not required, we own ipc_lock_object() */
__set_current_state(TASK_INTERRUPTIBLE);
ipc_unlock_object(&msq->q_perm);
@@ -1183,8 +1208,12 @@ static long do_msgrcv(int msqid, void __
* signal) it will either see the message and continue ...
*/
msg = READ_ONCE(msr_d.r_msg);
- if (msg != ERR_PTR(-EAGAIN))
+ if (msg != ERR_PTR(-EAGAIN)) {
+ /* see MSG_BARRIER for purpose/pairing */
+ smp_acquire__after_ctrl_dep();
+
goto out_unlock1;
+ }
/*
* ... or see -EAGAIN, acquire the lock to check the message
@@ -1192,7 +1221,7 @@ static long do_msgrcv(int msqid, void __
*/
ipc_lock_object(&msq->q_perm);
- msg = msr_d.r_msg;
+ msg = READ_ONCE(msr_d.r_msg);
if (msg != ERR_PTR(-EAGAIN))
goto out_unlock0;
_
Patches currently in -mm which might be from manfred@xxxxxxxxxxxxxxxx are
smp_mb__beforeafter_atomic-update-documentation.patch
ipc-mqueuec-update-document-memory-barriers.patch
ipc-msgc-update-and-document-memory-barriers.patch
ipc-semc-document-and-update-memory-barriers.patch
