Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers
On Fri, 11 Oct 2019, Manfred Spraul wrote: 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? What if we did the reference count explicitly, and then just use wake_q_add_safe()? That would avoid the extra barrier, __pipelined_op() would become: list_del(); get_task_struct(); wake_q_add_safe(); WRITE_ONCE(->state, STATE_READY); Thanks, Davidlohr
Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers
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(>list);
- wake_q_add(wake_q, receiver->task);
+ list_del(>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(>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
Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers
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.
Similarly imo, the 'state' should also need them for write, even if
under the lock -- consistency and documentation, for example.
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:
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(>list);
- wake_q_add(wake_q, receiver->task);
+ list_del(>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(>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,
[PATCH 2/5] ipc/mqueue.c: Update/document memory barriers
Update and document memory barriers for mqueue.c:
- ewp->state is read without any locks, thus READ_ONCE is required.
- add smp_aquire__after_ctrl_dep() after the RAED_ONCE, we need
acquire semantics if the value is STATE_READY.
- document that the code relies on the barrier inside wake_q_add()
- document why __set_current_state() may be used:
Reading task->state cannot happen before the wake_q_add() call,
which happens while holding info->lock.
Signed-off-by: Manfred Spraul
Cc: Waiman Long
Cc: Davidlohr Bueso
---
ipc/mqueue.c | 32 +---
1 file changed, 21 insertions(+), 11 deletions(-)
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 3d920ff15c80..902167407737 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -646,17 +646,25 @@ static int wq_sleep(struct mqueue_inode_info *info, int
sr,
wq_add(info, sr, ewp);
for (;;) {
+ /* memory barrier not required, we hold info->lock */
__set_current_state(TASK_INTERRUPTIBLE);
spin_unlock(>lock);
time = schedule_hrtimeout_range_clock(timeout, 0,
HRTIMER_MODE_ABS, CLOCK_REALTIME);
- if (ewp->state == STATE_READY) {
+ if (READ_ONCE(ewp->state) == STATE_READY) {
+ /*
+* Pairs, together with READ_ONCE(), with
+* the barrier in wake_q_add().
+*/
+ smp_acquire__after_ctrl_dep();
retval = 0;
goto out;
}
spin_lock(>lock);
+
+ /* we hold info->lock, so no memory barrier required */
if (ewp->state == STATE_READY) {
retval = 0;
goto out_unlock;
@@ -928,16 +936,11 @@ static inline void pipelined_send(struct wake_q_head
*wake_q,
{
receiver->msg = message;
list_del(>list);
+
wake_q_add(wake_q, receiver->task);
- /*
-* Rely on the implicit cmpxchg barrier from wake_q_add such
-* that we can ensure that updating receiver->state is the last
-* write operation: As once set, the receiver can continue,
-* and if we don't have the reference count from the wake_q,
-* yet, at that point we can later have a use-after-free
-* condition and bogus wakeup.
-*/
- receiver->state = STATE_READY;
+
+ /* The memory barrier is provided by wake_q_add(). */
+ WRITE_ONCE(receiver->state, STATE_READY);
}
/* pipelined_receive() - if there is task waiting in sys_mq_timedsend()
@@ -956,8 +959,11 @@ static inline void pipelined_receive(struct wake_q_head
*wake_q,
return;
list_del(>list);
+
wake_q_add(wake_q, sender->task);
- sender->state = STATE_READY;
+
+ /* The memory barrier is provided by wake_q_add(). */
+ WRITE_ONCE(sender->state, STATE_READY);
}
static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
@@ -1044,6 +1050,8 @@ static int do_mq_timedsend(mqd_t mqdes, const char __user
*u_msg_ptr,
} else {
wait.task = current;
wait.msg = (void *) msg_ptr;
+
+ /* memory barrier not required, we hold info->lock */
wait.state = STATE_NONE;
ret = wq_sleep(info, SEND, timeout, );
/*
@@ -1147,6 +1155,8 @@ static int do_mq_timedreceive(mqd_t mqdes, char __user
*u_msg_ptr,
ret = -EAGAIN;
} else {
wait.task = current;
+
+ /* memory barrier not required, we hold info->lock */
wait.state = STATE_NONE;
ret = wq_sleep(info, RECV, timeout, );
msg_ptr = wait.msg;
--
2.21.0
