[dm-devel] [PATCH 5/5] aio: Refactor aio_read_evt, use cmxchg(), fix bug

[Zach Brown]

> If libaio is the only thing in userspace looking at the ringbuffer, and
> if I'm looking at the latest libaio code this shouldn't break
> anything...

We can't assume that libaio is the only thing in userspace using the
mapped buffer -- as scary a thought as that is :).

If we wanted to change the behaviour of the ring we'd give userspace a
way to indicate that it accepts the new semantics.  I'm not immediately
sure how best to do that -- haven't given it much thought.

> Sticking the head and tail pointers on the same cacheline is one of my
> main complaints. If you could find that comment I'd be interested in
> reading it, though.

Yeah, that was on my list.. Heh, found it!

/*
* This structure defines the head of a ring of events that is mapped into user
* space.  It isn't used anymore because it has a number of design flaws.  I'll
* mention them here in the hopes that people read this when considering a
* design for a ring shared between user and kernel space.
*
* - The head and tail pointers are in the same cacheline.  This introduces
* false sharing between producers and consumers which could otherwise operate
* independent of one-another, presuming that the producers know ahead of time
* that room has been reserved for them.
*
* - The head and tail semantics are unconventional.  They're always less than
* the number of events in the ring and their meaning is reversed from the
* usual construct that one sees in, for example, ethernet hardware where the
* ring is a power of two and the indexes wrap but are masked when used to
* dereference elements.
*
* - Because of the weird head and tail semantics one can't distinguish from
* completely empty and full rings.  To work around this the ring is actually
* created with more events than the aio user asked for and that number is
* accounted for separately.  The number of free elements in the ring is
* greater than the number of operations that the kernel aio submission will
* allow.
*
* - Synchronizing head and tail updates between the kernel and user space
* requires a u32 cmpxchg.  futexes have infrastructure for this which requires
* working with user addresses.  Trying to nicely re-use the futex code leads
* to awkward code in aio which sometimes wants to work through kmap()
* addresses and other times want to work through the mmaped user space
* pointers.
*
* - The head and tail pointers are restricted in value to being less than the
* number of elements in the ring.  This means that a cmpxchg can see a index
* which has wrapped and confuse it for the index that it read before trying
* cmpxchg.  This can end up sending a duplicated previous event instead of a
* currently queued event in the worst case.
*
* - User space doesn't explicitly indicate that it wants to work with the ring
* from user space.  Even if user space never touches the ring the kernel still
* has pay the cost of the potential sharing with user space when inserting
* events.
*
* - The kernel magically maps it into the address space.  Users can't put it
* in whatever memory they like, like shared mem or hugetlb pages or tmpfs
* pages that they want to sendfile out of.
*
* - The ring is allocated out of unswappable kernel memory.  It would be
* better to have the ring allocated in userspace and given to the kernel.  In
* the common case the pages will be present and the overhead is minimal and
* complexity is kept out of the kernel.  In the worst case of memory pressure
* the kernel has fewer pinned pages tying its hands.
*/

The submission and completion interface that I threw together for the
acall experiments took all this into account:

 http://lwn.net/Articles/316806/

Man, this all brings back memories :).

- z