cvs commit: src/sys/kern subr_mbuf.c

[Bosko Milekic]

bmilekic    2003/06/03 12:19:13 PDT

  FreeBSD src repository

  Modified files:
    sys/kern             subr_mbuf.c 
  Log:
  Fix a potential bucket leak where when freeing to an empty bucket
  we failed to put the bucket back into the general cache/container.
  
  Also, fix a bad assumption.  There was a KASSERT() that aimed to
  guarantee that whenever the pcpu container's mc_starved was > 0,
  that whatever the bucket we were freeing to was an empty bucket,
  assuming it belonged to the pcpu container cache. However, there
  is at least one case where this is not true anymore; consider:
  1) All containers empty, next thread to try to alloc will touch
     a pcpu container, notice it's empty, and increment the pcpu
     container's mc_starved.
  2) Some other thread frees an mbuf belonging to a bucket in
     the general cache/container.  Then it frees another mbuf
     belonging to the same bucket (still in gen container).
  3) Some third thread tries to allocate an mbuf from the pcpu
     container and, since empty, grabs one mbuf now available
     in the general cache and moves the non-empty bucket from
     which it took 1 mbuf and to which the thread in (2) freed
     to, and moves it to the pcpu container.
  4) A final thread tries to free an mbuf belonging to the
     NON-EMPTY bucket mentionned in (2) and (3) and, since
     the pcpu container's mc_starved is > 0, but the bucket
     is obviously non-empty, it trips on the KASSERT.
  This meant that one could potentially get a panic in some
  cases when out of mbufs and clusters.  The problem could
  be mitigated by commenting out some cv_signal() calls,
  but I'm assuming that was pure coincidence and this is
  the correct fix.
  
  Revision  Changes    Path
  1.51      +29 -57    src/sys/kern/subr_mbuf.c