[Bug 1655225] Re: Under heavy load qemu hits bdrv_error_action: Assertion `error >= 0' failed

[ChristianEhrhardt]

qemu_2.0.0+dfsg-2ubuntu1.31 migrated tonight, and as I pointed out this
is already through review and all the tests that came to my mind on a
ppa base.

Since Chiluk still is in the application process to be able to SRU
upload I'll sponsor for now - I hope I did not miss that you got
approved and wanted to do on your own.

It is now in the unapproved queue for the SRU Team to take a look at.

@Chiluk - as you pointed out this is not really explicitly testable. But
if you could - once in proposed - run all sorts of stress against it
that would be great as a minimal verification-done.

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Title:
  Under heavy load qemu hits bdrv_error_action: Assertion `error >= 0'
  failed

Status in qemu package in Ubuntu:
  New
Status in qemu source package in Trusty:
  New

Bug description:
  [Impact]

   * VM running in QEMU with this message in qemu log
  qemu-system-x86_64: /build/qemu-_D3HGx/qemu-2.0.0+dfsg/block.c:3491: bdrv_error_action: Assertion `error >= 0' failed.

   * This results in the VM needing to be restarted.

  [Test Case]

   * This only reproduces under extremely high I/O load and very infrequently.
     Once per month on major clouds.

  [Regression Potential]

   * Commit 3bbf572345c is included in 2.4.0-rc4 and newer, and has not
  been removed in latest develepment. I have not found any commits
  referencing it as a regressor.

   * Patch adds memory barriers so regressions may show up in the form of
     performance issues.

  [Other Info]

   * Redhat identified this commit as the fix for the issue here
     https://access.redhat.com/solutions/1459913
     https://bugzilla.redhat.com/show_bug.cgi?id=1142857
     https://git.centos.org/blob/rpms!qemu-kvm/05bba06e575829071bce813e12709f9ec477f120/SOURCES!kvm-atomics-add-explicit-compiler-fence-in-__atomic-memo.patch
   * Only apply to Trusty as Xenial and newer have fix already.  Ignoring
     Precise because no currently reported cases.

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