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Date: Fri, 4 Sep 2015 15:49:37 -0400
From: Peter Hurley <peter@...leysoftware.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, jslaby@...e.com,
LKML <linux-kernel@...r.kernel.org>, Jiri Slaby <jslaby@...e.cz>,
Andrey Konovalov <andreyknvl@...gle.com>,
Kostya Serebryany <kcc@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
Hans Boehm <hboehm@...gle.com>
Subject: Re: [PATCH] tty: fix data races on tty_buffer.commit
On 09/04/2015 03:37 PM, Dmitry Vyukov wrote:
> On Fri, Sep 4, 2015 at 9:34 PM, Peter Hurley <peter@...leysoftware.com> wrote:
>> Hi Dmitry,
>>
>> On 09/04/2015 03:09 PM, Dmitry Vyukov wrote:
>>> Race on buffer data happens in the following scenario:
>>> __tty_buffer_request_room does a plain write of tail->commit,
>>> no barriers were executed before that.
>>> At this point flush_to_ldisc reads this new value of commit,
>>> and reads buffer data, no barriers in between.
>>> The committed buffer data is not necessary visible to flush_to_ldisc.
>>
>> Please submit one patch for each "fix", because it is not possible
>> to review what you believe you're fixing.
>>
>> See below for an example.
>>
>>> Similar bug happens when tty_schedule_flip commits data.
>>>
>>> Another race happens in tty_buffer_flush. It uses plain reads
>>> to read tty_buffer.next, as the result it can free a buffer
>>> which has pending writes in __tty_buffer_request_room thread.
>>> For example, tty_buffer_flush calls tty_buffer_free which
>>> reads b->size, the size may not be visible to this thread.
>>> As the result a large buffer can hang in the freelist.
>>>
>>> Update commit with smp_store_release and read commit with
>>> smp_load_acquire, as it is commit that signals data readiness.
>>> This is orthogonal to the existing synchronization on tty_buffer.next,
>>> which is required to not dismiss a buffer with unconsumed data.
>>>
>>> The data race was found with KernelThreadSanitizer (KTSAN).
>>>
>>> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
>>> ---
>>> drivers/tty/tty_buffer.c | 38 ++++++++++++++++++++++++--------------
>>> 1 file changed, 24 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
>>> index 4cf263d..4fae5d1 100644
>>> --- a/drivers/tty/tty_buffer.c
>>> +++ b/drivers/tty/tty_buffer.c
>>> @@ -89,7 +89,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
>>> struct tty_bufhead *buf = &port->buf;
>>> int restart;
>>>
>>> - restart = buf->head->commit != buf->head->read;
>>> + restart = READ_ONCE(buf->head->commit) != buf->head->read;
>>>
>>> atomic_dec(&buf->priority);
>>> mutex_unlock(&buf->lock);
>>> @@ -242,11 +242,14 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
>>> atomic_inc(&buf->priority);
>>>
>>> mutex_lock(&buf->lock);
>>> - while ((next = buf->head->next) != NULL) {
>>> + /* paired with smp_store_release in __tty_buffer_request_room();
>>> + * ensures there are no outstanding writes to buf->head when we free it
>>> + */
>>> + while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
>>> tty_buffer_free(port, buf->head);
>>> buf->head = next;
>>> }
>>> - buf->head->read = buf->head->commit;
>>> + buf->head->read = READ_ONCE(buf->head->commit);
>>>
>>> if (ld && ld->ops->flush_buffer)
>>> ld->ops->flush_buffer(tty);
>>> @@ -290,13 +293,15 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
>>> if (n != NULL) {
>>> n->flags = flags;
>>> buf->tail = n;
>>> - b->commit = b->used;
>>> - /* paired w/ barrier in flush_to_ldisc(); ensures the
>>> - * latest commit value can be read before the head is
>>> - * advanced to the next buffer
>>> + /* paired with smp_load_acquire in flush_to_ldisc();
>>> + * ensures flush_to_ldisc() sees buffer data.
>>> */
>>> - smp_wmb();
>>> - b->next = n;
>>> + smp_store_release(&b->commit, b->used);
>>> + /* paired with smp_load_acquire in flush_to_ldisc();
>>> + * ensures the latest commit value can be read before
>>> + * the head is advanced to the next buffer
>>> + */
>>> + smp_store_release(&b->next, n);
>>> } else if (change)
>>> size = 0;
>>> else
>>> @@ -394,7 +399,10 @@ void tty_schedule_flip(struct tty_port *port)
>>> {
>>> struct tty_bufhead *buf = &port->buf;
>>>
>>> - buf->tail->commit = buf->tail->used;
>>> + /* paired with smp_load_acquire in flush_to_ldisc(); ensures the
>>> + * committed data is visible to flush_to_ldisc()
>>> + */
>>> + smp_store_release(&buf->tail->commit, buf->tail->used);
>>> schedule_work(&buf->work);
>>
>> schedule_work() is an implied barrier for obvious reasons.
>
> OK, I will split.
> To answer this particular question: you need release/write barrier
> _before_ the synchronizing store, not _after_. Once the store to
> commit happened, another thread can start reading buffer data, this
> thread has not yet executed schedule_work at this point.
No.
If the work is already running, a new work will be scheduled, and the
new work will pick up the changed commit index.
If the work is already running /and it happens to see the new commit index/,
it will process the buffer. The new work will start and discover there is
nothing to do.
Regards,
Peter Hurley
PS - You need to base your patches on current mainline. You'll see that I already
converted the smp_rmb()/smp_wmb() of 'next' to load_acquire/store_release. FWIW,
that's not a fix, but a minor optimization. Commit sha 069f38b4983efaea9
>>> }
>>> EXPORT_SYMBOL(tty_schedule_flip);
>>> @@ -488,13 +496,15 @@ static void flush_to_ldisc(struct work_struct *work)
>>> if (atomic_read(&buf->priority))
>>> break;
>>>
>>> - next = head->next;
>>> - /* paired w/ barrier in __tty_buffer_request_room();
>>> + /* paired with smp_store_release in __tty_buffer_request_room();
>>> * ensures commit value read is not stale if the head
>>> * is advancing to the next buffer
>>> */
>>> - smp_rmb();
>>> - count = head->commit - head->read;
>>> + next = smp_load_acquire(&head->next);
>>> + /* paired with smp_store_release in __tty_buffer_request_room();
>>> + * ensures we see the committed buffer data
>>> + */
>>> + count = smp_load_acquire(&head->commit) - head->read;
>>> if (!count) {
>>> if (next == NULL) {
>>> check_other_closed(tty);
>>>
>>
>
>
>
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