[Libguestfs] nbdkit blocksize filter, read-modify-write, and concurrency

[Nikolaus Rath]

On May 24 2022, Eric Blake <eblake at redhat.com> wrote:
> 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:
>
> 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.

Yes, this is the scenario that I am worried about.

I this is a data corruption problem no matter if we assume that writes
should be atomically or not.

In this scenario, the client has issued exactly one request to write
(among other things) bytes 0-4. This request was executed successfully,
so bytes 0-4 should have the new contents. There was no other write that
affected this byte range, so whether the write was done atomic or not
does not matter.

> 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.


Yes, I was just picking a very small number to illustrate the problem. I
have seen this happen in practice with much larger blocksizes (32 kB+).

> 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?

How about a per-block lock as implemented for the S3 plugin in
https://gitlab.com/nbdkit/nbdkit/-/merge_requests/10?

It might be a bit harder to do in plain C because of the absence set
datatypes, but I think it's should work for the blocksize filter as well.


Best,
-Nikolaus
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