Messages in this thread | | | Subject | Re: [PATCH 2/5] ipc/mqueue.c: Update/document memory barriers | From | Manfred Spraul <> | Date | Fri, 11 Oct 2019 20:53:25 +0200 |
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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
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