id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
11968	Undefined behavior in `boost::lockfree::queue`: nodes are always misaligned	yury.zaytsev@…	timblechmann	"Here is a minimal reproducer to illustrate the problem:

{{{
#include <boost/lockfree/queue.hpp>

struct entry {
    int x = -1;
};

int main() {

    boost::lockfree::queue<entry, boost::lockfree::capacity<1>> queue;

    queue.push({0});

    entry d;
    queue.pop(d);

}
}}}

{{{
zaytsev@work:~/src/gcc$ g++ -std=c++14 -fsanitize=undefined lockfree.cpp 
zaytsev@work:~/src/gcc$ ./a.out 
/usr/include/boost/lockfree/detail/freelist.hpp:442:9: runtime error: constructor call on misaligned address 0x7fffd0831720 for type 'struct node', which requires 64 byte alignment
0x7fffd0831720: note: pointer points here
 00 00 00 00  00 00 a6 5b 2a 7f 00 00  ff ff 00 00 01 00 00 00  40 17 83 d0 ff 7f 00 00  f8 0a 40 00
              ^ 
/usr/include/boost/lockfree/detail/freelist.hpp:454:9: runtime error: constructor call on misaligned address 0x7fffd08316e0 for type 'struct node', which requires 64 byte alignment
0x7fffd08316e0: note: pointer points here
 ff 7f 00 00  02 00 60 00 00 00 00 00  70 08 40 00 00 00 00 00  88 50 60 00 00 00 00 00  d9 64 a9 5b
              ^ 
/usr/include/boost/lockfree/queue.hpp:102:19: runtime error: member access within misaligned address 0x7fffd08316e0 for type 'struct node', which requires 64 byte alignment
0x7fffd08316e0: note: pointer points here
 ff 7f 00 00  02 00 60 00 00 00 00 00  70 08 40 00 00 00 00 00  88 50 60 00 00 00 00 00  d9 64 a9 5b
              ^ 
}}}

This happens because nodes are declared to be cache line aligned in `queue.hpp`:

{{{
struct BOOST_LOCKFREE_CACHELINE_ALIGNMENT node { ... };
}}}

However, alignment to 64 bytes makes node an over-aligned type, and for such types the behavior is implementation defined. In particular, GCC 5.3, which was used for this test, doesn't seem to take alignment requirements of over-aligned types into account when allocating on the heap.

Apparently, nobody has noticed this so far, because on x86 this will simply cause a slowdown, but on other targets, such as ARM, the program would be terminated with a `SIGBUS` if a misaligned read occurs. It could be that the problem reported here: https://github.com/boostorg/lockfree/issues/11 is a manifestation of this larger issue.

Now, I would be happy to suggest a fix, but the problem is rather tricky.

1. If the storage for nodes is allocated on the heap, then probably the fix is as simple as replacing the default `std::allocator` with `boost::aligned_allocator`.

2. However, what makes `boost::lockfree::queue` particularly interesting is the support for compile-time sized storage. In this case, the situation is not so simple. One can get aligned storage with something like `boost::aligned_storage<size * sizeof(T), BOOST_LOCKFREE_CACHELINE_BYTES> data`, but is only part of the solution.[[br]][[br]]Indeed, `compiletime_sized_freelist_storage` should not only align `data`, but also over-allocate it, and the node allocator from the `fixed_size_freelist` should place nodes in `data` at appropriate positions so that not only the first, but also subsequent nodes remain aligned.

3. In addition to that, `BOOST_LOCKFREE_CACHELINE_BYTES` should be exposed publicly as a constant and those who are passing custom allocators to the `boost::lockfree::queue` should use that together with `boost::aligned_allocator_adaptor` to make sure that their allocators honor the desired alignment.

So, before investing any time in a patch, the problem has first to be discussed in more details... Maybe it also makes sense to change Boost.Lockfree to use Boost.Align, which was probably not used before because the former predated the latter.

For now, the easiest stop gap solution that I can think of would be to patch out the alignment requirement, as it simply doesn't work as expected, but potentially causes problems for other architectures as x86. This would be something I would suggest doing ASAP for Boost 1.61."	Bugs	new	To Be Determined	lockfree	Boost Development Trunk	Showstopper