Commits on Dec 19, 2014

1. mutex: force serialization on mutex_exit() to fix races …

   It is known that mutex in Linux is not safe when using it to synchronize
   the freeing of object in which the mutex is embedded:
   http://lwn.net/Articles/575477/
   
   The known places in ZFS which are suspected to suffer from the race condition
   are zio->io_lock and dbuf->db_mtx:
     zio uses zio->io_lock and zio->io_cv to synchronize freeing between zio_wait()
     and zio_done().
     dbuf uses dbuf->db_mtx to protect reference counting.
   
   Add it's been reported multiple time that zio->io_lock and dbuf->db_mtx suffer
   from corruption, which is exactly the symptom of the race.
   openzfs/zfs#2523
   openzfs/zfs#2897
   
   This patch fix this kind of race by forcing serializaion on mutex_exit() with
   a spinlock, making the mutex safer by sacrificing a bit of performance and
   memory overhead.
   
   Signed-off-by: Chunwei Chen <tuxoko@gmail.com>

   

   tuxoko committed Dec 19, 2014
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