1. 25 Jan, 2015 1 commit
  2. 20 Dec, 2013 1 commit
  3. 14 Nov, 2013 1 commit
  4. 15 Oct, 2013 1 commit
    • Steven Whitehouse's avatar
      GFS2: Use lockref for glocks · e66cf161
      Steven Whitehouse authored
      Currently glocks have an atomic reference count and also a spinlock
      which covers various internal fields, such as the state. This intent of
      this patch is to replace the spinlock and the atomic reference count
      with a lockref structure. This contains a spinlock which we can continue
      to use as before, and a reference counter which is used in conjuction
      with the spinlock to replace the previous atomic counter.
      
      As a result of this there are some new rules for reference counting on
      glocks. We need to distinguish between reference count changes under
      gl_spin (which are now just increment or decrement of the new counter,
      provided the count cannot hit zero) and those which are outside of
      gl_spin, but which now take gl_spin internally.
      
      The conversion is relatively straight forward. There is probably some
      further clean up which can be done, but the priority at this stage is to
      make the change in as simple a manner as possible.
      
      A consequence of this change is that the reference count is being
      decoupled from the lru list processing. This should allow future
      adoption of the lru_list code with glocks in due course.
      
      The reason for using the "dead" state and not just relying on 0 being
      the "invalid state" is so that in due course 0 ref counts can be
      allowable. The intent is to eventually be able to remove the ref count
      changes which are currently hidden away in state_change().
      Signed-off-by: default avatarSteven Whitehouse <swhiteho@redhat.com>
      e66cf161
  5. 07 Sep, 2013 1 commit
    • Linus Torvalds's avatar
      lockref: add ability to mark lockrefs "dead" · e7d33bb5
      Linus Torvalds authored
      The only actual current lockref user (dcache) uses zero reference counts
      even for perfectly live dentries, because it's a cache: there may not be
      any users, but that doesn't mean that we want to throw away the dentry.
      
      At the same time, the dentry cache does have a notion of a truly "dead"
      dentry that we must not even increment the reference count of, because
      we have pruned it and it is not valid.
      
      Currently that distinction is not visible in the lockref itself, and the
      dentry cache validation uses "lockref_get_or_lock()" to either get a new
      reference to a dentry that already had existing references (and thus
      cannot be dead), or get the dentry lock so that we can then verify the
      dentry and increment the reference count under the lock if that
      verification was successful.
      
      That's all somewhat complicated.
      
      This adds the concept of being "dead" to the lockref itself, by simply
      using a count that is negative.  This allows a usage scenario where we
      can increment the refcount of a dentry without having to validate it,
      and pushing the special "we killed it" case into the lockref code.
      
      The dentry code itself doesn't actually use this yet, and it's probably
      too late in the merge window to do that code (the dentry_kill() code
      with its "should I decrement the count" logic really is pretty complex
      code), but let's introduce the concept at the lockref level now.
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      e7d33bb5
  6. 02 Sep, 2013 3 commits
    • Linus Torvalds's avatar
      lockref: implement lockless reference count updates using cmpxchg() · bc08b449
      Linus Torvalds authored
      Instead of taking the spinlock, the lockless versions atomically check
      that the lock is not taken, and do the reference count update using a
      cmpxchg() loop.  This is semantically identical to doing the reference
      count update protected by the lock, but avoids the "wait for lock"
      contention that you get when accesses to the reference count are
      contended.
      
      Note that a "lockref" is absolutely _not_ equivalent to an atomic_t.
      Even when the lockref reference counts are updated atomically with
      cmpxchg, the fact that they also verify the state of the spinlock means
      that the lockless updates can never happen while somebody else holds the
      spinlock.
      
      So while "lockref_put_or_lock()" looks a lot like just another name for
      "atomic_dec_and_lock()", and both optimize to lockless updates, they are
      fundamentally different: the decrement done by atomic_dec_and_lock() is
      truly independent of any lock (as long as it doesn't decrement to zero),
      so a locked region can still see the count change.
      
      The lockref structure, in contrast, really is a *locked* reference
      count.  If you hold the spinlock, the reference count will be stable and
      you can modify the reference count without using atomics, because even
      the lockless updates will see and respect the state of the lock.
      
      In order to enable the cmpxchg lockless code, the architecture needs to
      do three things:
      
       (1) Make sure that the "arch_spinlock_t" and an "unsigned int" can fit
           in an aligned u64, and have a "cmpxchg()" implementation that works
           on such a u64 data type.
      
       (2) define a helper function to test for a spinlock being unlocked
           ("arch_spin_value_unlocked()")
      
       (3) select the "ARCH_USE_CMPXCHG_LOCKREF" config variable in its
           Kconfig file.
      
      This enables it for x86-64 (but not 32-bit, we'd need to make sure
      cmpxchg() turns into the proper cmpxchg8b in order to enable it for
      32-bit mode).
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      bc08b449
    • Linus Torvalds's avatar
      lockref: uninline lockref helper functions · 2f4f12e5
      Linus Torvalds authored
      They aren't very good to inline, since they already call external
      functions (the spinlock code), and we're going to create rather more
      complicated versions of them that can do the reference count updates
      locklessly.
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      2f4f12e5
    • Linus Torvalds's avatar
      lockref: add 'lockref_get_or_lock() helper · b3abd802
      Linus Torvalds authored
      This behaves like "lockref_get_not_zero()", but instead of doing nothing
      if the count was zero, it returns with the lock held.
      
      This allows callers to revalidate the lockref-protected data structure
      if required even if the count was zero to begin with, and possibly
      increment the count if it passes muster.
      
      In particular, the dentry code wants this when it wants to turn an
      RCU-protected dentry into a stable refcounted one: if the dentry count
      it zero, but the sequence number still validates the dentry, we can take
      a reference to it.
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      b3abd802
  7. 28 Aug, 2013 1 commit
    • Waiman Long's avatar
      Add new lockref infrastructure reference implementation · 0f8f2aaa
      Waiman Long authored
      This introduces a new "lockref" structure that supports the concept of
      lockless updates of reference counts that still honor an attached
      spinlock.
      
      NOTE! This reference implementation is not the optimized lockless
      version, rather it is the fallback implementation using standard
      spinlocks.  The actual optimized versions will be merged into 3.12, but
      I wanted to get the infrastructure in place and document the new
      interfaces.
      
      [ Also note that this particular commit is drastically cut-down minimal
        version of the original patch by Waiman.  In order to properly credit
        the original author I'm marking Waiman as the author here, but in the
        end this patch bears little resemblance to the patch by Waiman.  So
        blame any errors on me editing things down to the point where I can
        introduce the infrastructure before the merge window for 3.12 actually
        opens.     - Linus ]
      Signed-off-by: default avatarWaiman Long <Waiman.Long@hp.com>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      0f8f2aaa