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    • Linus Torvalds's avatar
      pipe: remove pipe_wait() and fix wakeup race with splice · 472e5b05
      Linus Torvalds authored 
      The pipe splice code still used the old model of waiting for pipe IO by
using a non-specific "pipe_wait()" that waited for any pipe event to
happen, which depended on all pipe IO being entirely serialized by the
pipe lock.  So by checking the state you were waiting for, and then
adding yourself to the wait queue before dropping the lock, you were
guaranteed to see all the wakeups.

Strictly speaking, the actual wakeups were not done under the lock, but
the pipe_wait() model still worked, because since the waiter held the
lock when checking whether it should sleep, it would always see the
current state, and the wakeup was always done after updating the state.

However, commit 0ddad21d ("pipe: use exclusive waits when reading or
writing") split the single wait-queue into two, and in the process also
made the "wait for event" code wait for _two_ wait queues, and that then
showed a race with the wakers that were not serialized by the pipe lock.

It's only splice that used that "pipe_wait()" model, so the problem
wasn't obvious, but Josef Bacik reports:

 "I hit a hang with fstest btrfs/187, which does a btrfs send into
  /dev/null. This works by creating a pipe, the write side is given to
  the kernel to write into, and the read side is handed to a thread that
  splices into a file, in this case /dev/null.

  The box that was hung had the write side stuck here [pipe_write] and
  the read side stuck here [splice_from_pipe_next -> pipe_wait].

  [ more details about pipe_wait() scenario ]

  The problem is we're doing the prepare_to_wait, which sets our state
  each time, however we can be woken up either with reads or writes. In
  the case above we race with the WRITER waking us up, and re-set our
  state to INTERRUPTIBLE, and thus never break out of schedule"

Josef had a patch that avoided the issue in pipe_wait() by just making
it set the state only once, but the deeper problem is that pipe_wait()
depends on a level of synchonization by the pipe mutex that it really
shouldn't.  And the whole "wait for any pipe state change" model really
isn't very good to begin with.

So rather than trying to work around things in pipe_wait(), remove that
legacy model of "wait for arbitrary pipe event" entirely, and actually
create functions that wait for the pipe actually being readable or
writable, and can do so without depending on the pipe lock serializing
everything.

Fixes: 0ddad21d ("pipe: use exclusive waits when reading or writing")
Link: https://lore.kernel.org/linux-fsdevel/bfa88b5ad6f069b2b679316b9e495a970130416c.1601567868.git.josef@toxicpanda.com/


Reported-by: default avatarJosef Bacik <josef@toxicpanda.com>
Reviewed-and-tested-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      472e5b05
    • Linus Torvalds's avatar
      Merge tag 'iommu-fixes-v5.9-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu · 44b6e23b
      Linus Torvalds authored 
      Pull iommu fixes from Joerg Roedel:

 - Fix a device reference counting bug in the Exynos IOMMU driver.

 - Lockdep fix for the Intel VT-d driver.

 - Fix a bug in the AMD IOMMU driver which caused corruption of the IVRS
   ACPI table and caused IOMMU driver initialization failures in kdump
   kernels.

* tag 'iommu-fixes-v5.9-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu:
  iommu/vt-d: Fix lockdep splat in iommu_flush_dev_iotlb()
  iommu/amd: Fix the overwritten field in IVMD header
  iommu/exynos: add missing put_device() call in exynos_iommu_of_xlate()
      44b6e23b
    • Linus Torvalds's avatar
      Merge tag 'arm64-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux · eed2ef44
      Linus Torvalds authored 
      Pull arm64 fix from Catalin Marinas:
 "A previous commit to prevent AML memory opregions from accessing the
  kernel memory turned out to be too restrictive. Relax the permission
  check to permit the ACPI core to map kernel memory used for table
  overrides"

* tag 'arm64-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux:
  arm64: permit ACPI core to map kernel memory used for table overrides
      eed2ef44