File System and File System Filter Ecosystem Update

1
File System and File System Filter Ecosystem
Update

• Neal Christiansen
• Development Lead
• File System Filter Team

2
BOOT.INI and Longhorn

• The boot.ini file is no longer used to configure
  boot parameters.
• New BCD (boot configuration data) file
• Implemented as a registry hive
• Resides in \boot\bcd
• There is a new tool called bcdedit which is used
  to configure options
• Boot.ini is still used for downlevel operating
  systems if you are booting multiple OSs on the
  same system

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Useful bcdedit commands

• bcdedit enum
• List current settings
• bcdedit enum all
• List all settings
• bcdedit /?
• Basic help
• bcdedit /? Types
• Lists the bulk of the settings

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Useful bcdedit commands

• Enable debugging
• bcdedit -debug on
• Followed by one of the following
• bcdedit -dbgsettings serial debugport1
  baudrate115200
• bcdedit -dbgsettings 1394 CHANNEL32
• bcdedit dbgsettings USB TARGETNAMEU1

5
Doing IO from Completion Routines

• Many of you issue IO operations from IO
  Completion routines (or post-operation callbacks)
  when those routines are at an IRQL level lower
  then DPC level
• Filter manager promoted this with routines like
  FltDoCompletionProcessingWhenSafe

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Doing IO from Completion Routines

• We have determined that it is not safe to ever
  perform any operation in a completion (or
  post-operation) callback routine regardless of
  your current IRQL level
• We have been seeing deadlock with various storage
  drivers because of this
• The most common scenario we have seen is when
  using dynamic disks with RAID enabled
• We have seen it with other drivers as well
• Consider this a new rule for filters

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Doing IO from Completion Routines

• It is our very strong recommendation that you
  modify your existing products across all OSs to
  stop doing this
• You have two options
• If the operation is synchronous you should
  synchronize back to your dispatch routine
• Use IoIsOperationSynchronous
• If the operation is asynchronous you should queue
  the request to a worker thread
• You may have to throttle your worker thread usage
  because you can cause deadlocks if you consume
  all of the worker threads

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Doing IO from Completion Routines

• What is filter manager going to do about this?
• FltDoCompletionProcessingWhenSafe will be
  modified to either queue the request or
  automatically synchronize and call the specified
  routine.
• This will happen automatically so if you use the
  current routine you should not have to change
  anything
• We are going to modify the minifilter verifier to
  detect this situation and break