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Subject: Re: [boost] threadpool lockfree_channel
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2009-03-16 08:09:39

Tim Blechmann <tim_at_[hidden]> writes:

>>>> Suppose there are two threads calling deqeue() concurrently. If there is
>>>> an item in the queue, both will proceed identically down to line 137, as
>>>> neither thread modifies the queue data before then. Now suppose one
>>>> thread (A) gets preempted, but the other (B) proceeds. Thread B will
>>>> read next->data and assign it to ret. It will then update head_ with CAS
>>>> (which will succeed as no other thread is modifying/has modified the
>>>> data structure), and *deallocate the node* at line 141. This will
>>>> destroy next->data. Now, when thread A wakes up it will try and read
>>>> next->data => reading destroyed object, undefined behaviour.
>>> dealloc_node in line 141 does not free the node, but pushes it to a
>>> freelist stack ... from my understanding, this leads to an increased
>>> memory usage ... allocated nodes are not freed until the fifo is
>>> destroyed, but reused for new nodes ...
>>> so memory reclamation should be safe ... or am i missing something?
>> Your dealloc_node is:
>> void dealloc_node(node * n)
>> {
>> n->~node();
>> pool.deallocate(n);
>> }
>> So the node is destroyed (by the destructor) even if the memory is not
>> deallocated.
> i see your point ... restricting the use of the fifo queue to PODs
> should work around this issue ...

Yes, in that you can remove the node destructor from dealloc_node, and
thus you no longer have undefined behaviour.

However, the node could still be reused by another thread calling
push() before our thread A wakes up. In this case, you could have an
ABA problem --- thread A has a pointer to node X which originally had
a next ptr of Y, but in the mean time thread B has deallocated node
X. Node X may subsequently be reused with a next ptr of node Z. If it
node X becomes the head node before thread A wakes up, then the CAS on
line 139 will succeed, despite the fact that node X has been
deallocated and reallocated since, and the CAS will store the wrong
"next" value (Y rather than Z), thus corrupting the queue.

Thus, you cannot reuse node X unless you can be sure there is no
thread in the position of thread A. With the code as it stands, you
cannot know that, as thread A has not done any writing in dequeue().

Since you are using tagged pointers, you could increment the tag value
whenever you reuse a node, which will at least ensure that the CAS in
thread A fails if there is an ABA issue, since the tag part of the
pointer will be different. Since the tag has a finite size, this is
still a problem in theory, as the tag value might wrap around before
thread A resumes, but in practice it's probably OK.

Alternatively, you could use hazard pointers, reference-counted
pointers (as per the implementation I posted), reference counting on
the whole queue (last one out deletes any dequeued nodes), or another
memory reclamation scheme.


Anthony Williams
Author of C++ Concurrency in Action |
Custom Software Development |
Just Software Solutions Ltd, Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK

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