1. 14 Jul, 2017 1 commit
  2. 26 Sep, 2012 2 commits
    • Mikulas Patocka's avatar
      percpu-rw-semaphore: fix documentation typos · e6b5c082
      Mikulas Patocka authored
      One more patch for this thing, fixing some typos in the documentation.
      Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
      Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
      e6b5c082
    • Mikulas Patocka's avatar
      blockdev: turn a rw semaphore into a percpu rw semaphore · 62ac665f
      Mikulas Patocka authored
      This avoids cache line bouncing when many processes lock the semaphore
      for read.
      
      New percpu lock implementation
      
      The lock consists of an array of percpu unsigned integers, a boolean
      variable and a mutex.
      
      When we take the lock for read, we enter rcu read section, check for a
      "locked" variable. If it is false, we increase a percpu counter on the
      current cpu and exit the rcu section. If "locked" is true, we exit the
      rcu section, take the mutex and drop it (this waits until a writer
      finished) and retry.
      
      Unlocking for read just decreases percpu variable. Note that we can
      unlock on a difference cpu than where we locked, in this case the
      counter underflows. The sum of all percpu counters represents the number
      of processes that hold the lock for read.
      
      When we need to lock for write, we take the mutex, set "locked" variable
      to true and synchronize rcu. Since RCU has been synchronized, no
      processes can create new read locks. We wait until the sum of percpu
      counters is zero - when it is, there are no readers in the critical
      section.
      Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
      Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
      62ac665f