On 10/11/19 6:55 PM, Davidlohr Bueso wrote:
On Fri, 11 Oct 2019, Manfred Spraul wrote:
Update and document memory barriers for mqueue.c:
- ewp->state is read without any locks, thus READ_ONCE is required.
In general we relied on the barrier for not needing READ/WRITE_ONCE,
but I agree this scenario should be better documented with them.
After reading core-api/atomic_ops.rst:
> _ONCE() should be used. [...] Alternatively, you can place a barrier.
So both approaches are ok.
Let's follow the "should", i.e.: all operations on the ->state variables
to READ_ONCE()/WRITE_ONCE().
Then we have a standard, and since we can follow the "should", we should
do that.
Similarly imo, the 'state' should also need them for write, even if
under the lock -- consistency and documentation, for example.
Ok, so let's convert everything to _ONCE. (assuming that my analysis
below is incorrect)
In addition, I think it makes sense to encapsulate some of the
pipelined send/recv operations, that also can allow us to keep
the barrier comments in pipelined_send(), which I wonder why
you chose to remove. Something like so, before your changes:
I thought that the simple "memory barrier is provided" is enough, so I
had removed the comment.
But you are right, there are two different scenarios:
1) thread already in another wake_q, wakeup happens immediately after
the cmpxchg_relaxed().
This scenario is safe, due to the smp_mb__before_atomic() in wake_q_add()
2) thread woken up but e.g. a timeout, see ->state=STATE_READY, returns
to user space, calls sys_exit.
This must not happen before get_task_struct acquired a reference.
And this appears to be unsafe: get_task_struct() is refcount_inc(),
which is refcount_inc_checked(), which is according to lib/refcount.c
fully unordered.
Thus: ->state=STATE_READY can execute before the refcount increase.
Thus: ->state=STATE_READY needs a smp_store_release(), correct?
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 3d920ff15c80..be48c0ba92f7 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -918,17 +918,12 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *,
u_name)
* The same algorithm is used for senders.
*/
-/* pipelined_send() - send a message directly to the task waiting in
- * sys_mq_timedreceive() (without inserting message into a queue).
- */
-static inline void pipelined_send(struct wake_q_head *wake_q,
+static inline void __pipelined_op(struct wake_q_head *wake_q,
struct mqueue_inode_info *info,
- struct msg_msg *message,
- struct ext_wait_queue *receiver)
+ struct ext_wait_queue *this)
{
- receiver->msg = message;
- list_del(&receiver->list);
- wake_q_add(wake_q, receiver->task);
+ list_del(&this->list);
+ wake_q_add(wake_q, this->task);
/*
* Rely on the implicit cmpxchg barrier from wake_q_add such
* that we can ensure that updating receiver->state is the last
@@ -937,7 +932,19 @@ static inline void pipelined_send(struct
wake_q_head *wake_q,
* yet, at that point we can later have a use-after-free
* condition and bogus wakeup.
*/
- receiver->state = STATE_READY;
+ this->state = STATE_READY;
+}
+
+/* pipelined_send() - send a message directly to the task waiting in
+ * sys_mq_timedreceive() (without inserting message into a queue).
+ */
+static inline void pipelined_send(struct wake_q_head *wake_q,
+ struct mqueue_inode_info *info,
+ struct msg_msg *message,
+ struct ext_wait_queue *receiver)
+{
+ receiver->msg = message;
+ __pipelined_op(wake_q, info, receiver);
}
/* pipelined_receive() - if there is task waiting in sys_mq_timedsend()
@@ -955,9 +962,7 @@ static inline void pipelined_receive(struct
wake_q_head *wake_q,
if (msg_insert(sender->msg, info))
return;
- list_del(&sender->list);
- wake_q_add(wake_q, sender->task);
- sender->state = STATE_READY;
+ __pipelined_op(wake_q, info, sender);
}
static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
I would merge that into the series, ok?
--
Manfred