Libguestfs - May 2022 - nbdkit blocksize filter, read-modify-write, and concurrency

Hi,

How does the blocksize filter take into account writes that end-up overlapping
due to read-modify-write cycles?

Specifically, suppose there are two non-overlapping writes handled by two
different threads, that, due to blocksize requirements, overlap when expanded. I
think there is a risk that one thread may partially undo the work of the other
here.

Looking at the code, it seems that writes of unaligned heads and tails are
protected with a global lock.,
but writes of aligned data can occur concurrently.

However, does this not miss the case where there is one unaligned write that
overlaps with an aligned one?

For example, with blocksize 10, we could have:

Thread 1: receives write request for offset=0, size=10
Thread 2: receives write request for offset=4, size=16
Thread 1: acquires lock, reads bytes 0-4
Thread 2: does aligned write (no locking needed), writes bytes 0-10
Thread 1: writes bytes 0-10, overwriting data from Thread 2


Best,
-Nikolaus

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On Sat, May 21, 2022 at 01:21:11PM +0100, Nikolaus Rath
wrote:
> Hi,
>
> How does the blocksize filter take into account writes that end-up
> overlapping due to read-modify-write cycles?
>
> Specifically, suppose there are two non-overlapping writes handled
> by two different threads, that, due to blocksize requirements,
> overlap when expanded.  I think there is a risk that one thread may
> partially undo the work of the other here.
>
> Looking at the code, it seems that writes of unaligned heads and
> tails are protected with a global lock., but writes of aligned data
> can occur concurrently.
I agree.

Assuming the underlying plugin is NBDKIT_THREAD_MODEL_PARALLEL and no
other filters impose thread model limits, the blocksize filter does
not limit the thread model, so the thread model of nbdkit would also
be NBDKIT_THREAD_MODEL_PARALLEL.

That means that two writes either on different connections or
pipelined on the same connection could happen at the same time.
?blocksize_pwrite? would be called concurrently for the two requests.
> However, does this not miss the case where there is one unaligned
> write that overlaps with an aligned one?
>
> For example, with blocksize 10, we could have:
> 
> Thread 1: receives write request for offset=0, size=10
> Thread 2: receives write request for offset=4, size=16
> Thread 1: acquires lock, reads bytes 0-4
> Thread 2: does aligned write (no locking needed), writes bytes 0-10
> Thread 1: writes bytes 0-10, overwriting data from Thread 2
I believe this analysis is correct.  (CC'd to Eric who knows a lot
more about this.)

However I don't think it's a bug.  If a client doesn't want writes
to
squash each other, then it shouldn't send overlapping requests.  I bet
the same thing happens with an SSD.

NBD_CMD_FLAG_FUA is provided for clients that wish to ensure that a
write has been committed before sending another request.

Do you have an example of a client which sends overlapping requests
and depends on particular behaviour of the server?  You may be able to
get it to work by using nbdkit-noparallel-filter which can be used to
serialize nbdkit.

Rich.

-- 
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Read my programming and virtualization blog: http://rwmj.wordpress.com
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On Sat, May 21, 2022 at 01:21:11PM +0100, Nikolaus Rath
wrote:
> Hi,
> 
> How does the blocksize filter take into account writes that end-up
overlapping due to read-modify-write cycles?
> 
> Specifically, suppose there are two non-overlapping writes handled by two
different threads, that, due to blocksize requirements, overlap when expanded. I
think there is a risk that one thread may partially undo the work of the other
here.
> 
> Looking at the code, it seems that writes of unaligned heads and tails are
protected with a global lock.,
> but writes of aligned data can occur concurrently.
> 
> However, does this not miss the case where there is one unaligned write
that overlaps with an aligned one?
> 
> For example, with blocksize 10, we could have:
The blocksize filter requires blocks to be sized as a power of 2,
which 10 is not.  I will try to restate your question using hex
boundaries; please correct me if I'm mis-stating things.
> 
> Thread 1: receives write request for offset=0, size=10
> Thread 2: receives write request for offset=4, size=16
minblock = 0x10
Thread 1: receives write request for offset 0x00, size 0x10 (aligned request)
Thread 2: receives write request for offset 0x04, size 0x16 (unaligned offset,
unaligned size)

Graphically, we are wanting to write the following, given initial disk
contents of I:

       0   0   0   0   1   1   1   1
       0...4...8...a...0...4...8...a...
start      IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
T1:    AAAAAAAAAAAAAAAA
T2:        BBBBBBBBBBBBBBBBBBBBBB

Because both writes are issued simultaneously, we do not know whether
bytes 0x04 through 0x0f will be written as A or B.  But our assumption
is that because blocks are written atomically, we hope to get exactly
one of the two following results, where either T1 beat T2:

       0   0   0   0   1   1   1   1
       0...4...8...a...0...4...8...a...
end1:  AAAABBBBBBBBBBBBBBBBBBBBBBIIIIII

or where T2 beat T1:

       0   0   0   0   1   1   1   1
       0...4...8...a...0...4...8...a...
end2:  AAAAAAAAAAAAAAAABBBBBBBBBBIIIIII

However, you are worried that a third possibility occurs:
> Thread 1: acquires lock, reads bytes 0-4
> Thread 2: does aligned write (no locking needed), writes bytes 0-10
> Thread 1: writes bytes 0-10, overwriting data from Thread 2
These do not match your initial conditions above (why is thread 1
reading; why is thread 2 doing aligned action).  Again, I'm rewriting
this into something that I think matches what you intended to ask, but
I welcome corrections:

T2 sees that it needs to do RMW, grabs the lock, and reads 0x00-0x0f
for the unaligned head (it only needs 0x00-0x03, but we have to read a
block at a time), to populate its buffer with IIIIBBBBBBBBBBBB.

T1 now writes 0x00-0x0f with AAAAAAAAAAAAAAAA, without any lock
blocking it.

T2 now writes 0x00-0x0f using the contents of its buffer, resulting in:

       0   0   0   0   1   1   1   1
       0...4...8...a...0...4...8...a...
end3:  IIIIBBBBBBBBBBBBBBBBBBBBBBIIIIII

which does NOT reflect either of the possibilities where T1 and T2
write atomically.  Basically, we have become the victim of sharding.

You are correct that it is annoying that this third possibility (where
T1 appears to have never run) is possible with the blocksize filter.
And we should probably consider it as a data-corruption bug.  Your
blocksize example of 10 (or 0x10) bytes is unlikely, but we are more
likely to hit scenarios where an older guest assumes it is writing to
512-byte aligned hardware, while using the blocksize filter to try and
guarantee RMW atomic access to 4k modern hardware.  The older client
will be unaware that it must avoid parallel writes that are
512-aligned but land in the same 4k page, so it seems like the
blocksize filter should be doing that.

You have just demonstrated that our current approach (grabbing a
single semaphore, only around the unaligned portions), does not do
what we hoped.  So what WOULD protect us, while still allowing as much
parallelism as possible?  It sounds like we want a RW-lock (note: not
in the sense of parallel pread and exclusive pwrite operations, but
rather in the sense of parallel aligned operations taking a rdlock
whether it is a pread or pwrite operation, and exclusive unaligned
operations taking a wrlock whether pread or pwrite).

pthread_rwlock_t would work, although it does not have guarantees on
whether the implementation will favor readers or writers.  It is also
possible to implement our own using 2 mutexes (favors readers), or a
condvar and mutex (easy to favor writers) [1].  Favoring readers can
starve writers indefinitely but gets the most concurrency; favoring
writers avoids starvation but has less performance.  Other policies
are RCU (wait-free for rdlock) or seqlock, borrowing ideas from the
Linux kernel.  Maybe we make it a configurable knob which lock policy
to use, based on how likely the client is to do unaligned operations
that require a write lock?

[1] https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock


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