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author | Linus Torvalds <torvalds@linux-foundation.org> | 2013-09-02 12:12:15 -0700 |
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committer | Maciej Wereski <m.wereski@partner.samsung.com> | 2015-03-18 09:47:12 +0100 |
commit | e627a2a8ab89957b7beb6b70dec64481c48dbb81 (patch) | |
tree | b4664713eda48e4868854e49cb07822475f20510 /lib | |
parent | 8ff51b75dc12ab5c5114311a7002a7e19e642791 (diff) | |
download | linux-3.10-e627a2a8ab89957b7beb6b70dec64481c48dbb81.tar.gz linux-3.10-e627a2a8ab89957b7beb6b70dec64481c48dbb81.tar.bz2 linux-3.10-e627a2a8ab89957b7beb6b70dec64481c48dbb81.zip |
lockref: implement lockless reference count updates using cmpxchg()
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: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib')
-rw-r--r-- | lib/Kconfig | 10 | ||||
-rw-r--r-- | lib/lockref.c | 60 |
2 files changed, 69 insertions, 1 deletions
diff --git a/lib/Kconfig b/lib/Kconfig index fe01d418b09..62581eaa682 100644 --- a/lib/Kconfig +++ b/lib/Kconfig @@ -45,6 +45,16 @@ config STMP_DEVICE config PERCPU_RWSEM boolean +config ARCH_USE_CMPXCHG_LOCKREF + bool + +config CMPXCHG_LOCKREF + def_bool y if ARCH_USE_CMPXCHG_LOCKREF + depends on SMP + depends on !GENERIC_LOCKBREAK + depends on !DEBUG_SPINLOCK + depends on !DEBUG_LOCK_ALLOC + config CRC_CCITT tristate "CRC-CCITT functions" help diff --git a/lib/lockref.c b/lib/lockref.c index a9a4f4e1eff..7819c2d1d31 100644 --- a/lib/lockref.c +++ b/lib/lockref.c @@ -1,6 +1,33 @@ #include <linux/export.h> #include <linux/lockref.h> +#ifdef CONFIG_CMPXCHG_LOCKREF + +/* + * Note that the "cmpxchg()" reloads the "old" value for the + * failure case. + */ +#define CMPXCHG_LOOP(CODE, SUCCESS) do { \ + struct lockref old; \ + BUILD_BUG_ON(sizeof(old) != 8); \ + old.lock_count = ACCESS_ONCE(lockref->lock_count); \ + while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) { \ + struct lockref new = old, prev = old; \ + CODE \ + old.lock_count = cmpxchg(&lockref->lock_count, \ + old.lock_count, new.lock_count); \ + if (likely(old.lock_count == prev.lock_count)) { \ + SUCCESS; \ + } \ + } \ +} while (0) + +#else + +#define CMPXCHG_LOOP(CODE, SUCCESS) do { } while (0) + +#endif + /** * lockref_get - Increments reference count unconditionally * @lockcnt: pointer to lockref structure @@ -10,6 +37,12 @@ */ void lockref_get(struct lockref *lockref) { + CMPXCHG_LOOP( + new.count++; + , + return; + ); + spin_lock(&lockref->lock); lockref->count++; spin_unlock(&lockref->lock); @@ -23,9 +56,18 @@ EXPORT_SYMBOL(lockref_get); */ int lockref_get_not_zero(struct lockref *lockref) { - int retval = 0; + int retval; + + CMPXCHG_LOOP( + new.count++; + if (!old.count) + return 0; + , + return 1; + ); spin_lock(&lockref->lock); + retval = 0; if (lockref->count) { lockref->count++; retval = 1; @@ -43,6 +85,14 @@ EXPORT_SYMBOL(lockref_get_not_zero); */ int lockref_get_or_lock(struct lockref *lockref) { + CMPXCHG_LOOP( + new.count++; + if (!old.count) + break; + , + return 1; + ); + spin_lock(&lockref->lock); if (!lockref->count) return 0; @@ -59,6 +109,14 @@ EXPORT_SYMBOL(lockref_get_or_lock); */ int lockref_put_or_lock(struct lockref *lockref) { + CMPXCHG_LOOP( + new.count--; + if (old.count <= 1) + break; + , + return 1; + ); + spin_lock(&lockref->lock); if (lockref->count <= 1) return 0; |