Messages in this thread Patch in this message | | | From | Manfred Spraul <> | Subject | [PATCH 3/6] ipc/mqueue.c: Update/document memory barriers | Date | Sat, 12 Oct 2019 07:49:55 +0200 |
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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
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