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Message-ID: <5e08cb89-563c-4763-dd88-94edaf9d883b@colorfullife.com>
Date: Fri, 11 Oct 2019 20:53:25 +0200
From: Manfred Spraul <manfred@...orfullife.com>
To: Davidlohr Bueso <dave@...olabs.net>
Cc: LKML <linux-kernel@...r.kernel.org>,
Waiman Long <longman@...hat.com>, 1vier1@....de,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Jonathan Corbet <corbet@....net>
Subject: 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(&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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