lkml.org 
[lkml]   [2019]   [Oct]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 3/6] ipc/mqueue.c: Update/document memory barriers
Date
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 READ_ONCE, we need
acquire semantics if the value is STATE_READY.

- add an explicit memory barrier to __pipelined_op(), the
refcount must have been increased before the updated state becomes
visible

- 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. Thus the spin_unlock()
is the RELEASE, and the spin_lock() is the ACQUIRE.

For completeness: there is also a 3 CPU szenario, if the to be woken
up task is already on another wake_q.
Then:
- CPU1: spin_unlock() of the task that goes to sleep is the RELEASE
- CPU2: the spin_lock() of the waker is the ACQUIRE
- CPU2: smp_mb__before_atomic inside wake_q_add() is the RELEASE
- CPU3: smp_mb__after_spinlock() inside try_to_wake_up() is the ACQUIRE

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
ipc/mqueue.c | 32 +++++++++++++++++++++-----------
1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index be48c0ba92f7..b80574822f0a 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -646,18 +646,26 @@ 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(&info->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 __pipelined_op().
+ */
+ smp_acquire__after_ctrl_dep();
retval = 0;
goto out;
}
spin_lock(&info->lock);
- if (ewp->state == STATE_READY) {
+
+ /* we hold info->lock, so no memory barrier required */
+ if (READ_ONCE(ewp->state) == STATE_READY) {
retval = 0;
goto out_unlock;
}
@@ -925,14 +933,12 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
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
- * 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.
+ * The barrier is required to ensure that the refcount increase
+ * inside wake_q_add() is completed before the state is updated.
+ *
+ * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
*/
- this->state = STATE_READY;
+ smp_store_release(&this->state, STATE_READY);
}

/* pipelined_send() - send a message directly to the task waiting in
@@ -1049,7 +1055,9 @@ static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
} else {
wait.task = current;
wait.msg = (void *) msg_ptr;
- wait.state = STATE_NONE;
+
+ /* memory barrier not required, we hold info->lock */
+ WRITE_ONCE(wait.state, STATE_NONE);
ret = wq_sleep(info, SEND, timeout, &wait);
/*
* wq_sleep must be called with info->lock held, and
@@ -1152,7 +1160,9 @@ static int do_mq_timedreceive(mqd_t mqdes, char __user *u_msg_ptr,
ret = -EAGAIN;
} else {
wait.task = current;
- wait.state = STATE_NONE;
+
+ /* memory barrier not required, we hold info->lock */
+ WRITE_ONCE(wait.state, STATE_NONE);
ret = wq_sleep(info, RECV, timeout, &wait);
msg_ptr = wait.msg;
}
--
2.21.0
\
 
 \ /
  Last update: 2019-10-12 07:51    [W:0.213 / U:0.164 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site