• 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
Kconfig 9.46 KB