1. 30 Apr, 2008 1 commit
    • Thomas Gleixner's avatar
      infrastructure to debug (dynamic) objects · 3ac7fe5a
      Thomas Gleixner authored
      We can see an ever repeating problem pattern with objects of any kind in the
      kernel:
      
      1) freeing of active objects
      2) reinitialization of active objects
      
      Both problems can be hard to debug because the crash happens at a point where
      we have no chance to decode the root cause anymore.  One problem spot are
      kernel timers, where the detection of the problem often happens in interrupt
      context and usually causes the machine to panic.
      
      While working on a timer related bug report I had to hack specialized code
      into the timer subsystem to get a reasonable hint for the root cause.  This
      debug hack was fine for temporary use, but far from a mergeable solution due
      to the intrusiveness into the timer code.
      
      The code further lacked the ability to detect and report the root cause
      instantly and keep the system operational.
      
      Keeping the system operational is important to get hold of the debug
      information without special debugging aids like serial consoles and special
      knowledge of the bug reporter.
      
      The problems described above are not restricted to timers, but timers tend to
      expose it usually in a full system crash.  Other objects are less explosive,
      but the symptoms caused by such mistakes can be even harder to debug.
      
      Instead of creating specialized debugging code for the timer subsystem a
      generic infrastructure is created which allows developers to verify their code
      and provides an easy to enable debug facility for users in case of trouble.
      
      The debugobjects core code keeps track of operations on static and dynamic
      objects by inserting them into a hashed list and sanity checking them on
      object operations and provides additional checks whenever kernel memory is
      freed.
      
      The tracked object operations are:
      - initializing an object
      - adding an object to a subsystem list
      - deleting an object from a subsystem list
      
      Each operation is sanity checked before the operation is executed and the
      subsystem specific code can provide a fixup function which allows to prevent
      the damage of the operation.  When the sanity check triggers a warning message
      and a stack trace is printed.
      
      The list of operations can be extended if the need arises.  For now it's
      limited to the requirements of the first user (timers).
      
      The core code enqueues the objects into hash buckets.  The hash index is
      generated from the address of the object to simplify the lookup for the check
      on kfree/vfree.  Each bucket has it's own spinlock to avoid contention on a
      global lock.
      
      The debug code can be compiled in without being active.  The runtime overhead
      is minimal and could be optimized by asm alternatives.  A kernel command line
      option enables the debugging code.
      
      Thanks to Ingo Molnar for review, suggestions and cleanup patches.
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
      Cc: Greg KH <greg@kroah.com>
      Cc: Randy Dunlap <randy.dunlap@oracle.com>
      Cc: Kay Sievers <kay.sievers@vrfy.org>
      Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      3ac7fe5a