[PATCH] blockgroup_lock: hashed spinlocks for ext2 and ext3

ext2 and ext3 per-blockgroup metadata needs locking.  An fs-wide lock is
expensive, and a per-blockgroup lock consumes too much storage (up to 32768
blockgroups per filesystem).  We need something in-between.

blockgroup_locks are very simple hashed spinlocks which provide this
compromise.  The size of the lock is scaled by NR_CPUS to implement an
additional speed/space tradeoff.

These locks are actually fairly generic.  However I presented it as something
which is specific to ext2 and ext3 so that people wouldn't go using them all
over the place.  They consume a lot of storage.

include/linux/blockgroup_lock.h

+/*
+ * Per-blockgroup locking for ext2 and ext3.
+ *
+ * Simple hashed spinlocking.
+ */
+
+#include <linux/config.h>
+#include <linux/spinlock.h>
+#include <linux/cache.h>
+
+#ifdef CONFIG_SMP
+
+/*
+ * We want a power-of-two.  Is there a better way than this?
+ */
+
+#if NR_CPUS >= 32
+#define NR_BG_LOCKS	128
+#elif NR_CPUS >= 16
+#define NR_BG_LOCKS	64
+#elif NR_CPUS >= 8
+#define NR_BG_LOCKS	32
+#elif NR_CPUS >= 4
+#define NR_BG_LOCKS	16
+#elif NR_CPUS >= 2
+#define NR_BG_LOCKS	8
+#else
+#define NR_BG_LOCKS	4
+#endif
+
+#else	/* CONFIG_SMP */
+#define NR_BG_LOCKS	1
+#endif	/* CONFIG_SMP */
+
+struct bgl_lock {
+	spinlock_t lock;
+} ____cacheline_aligned_in_smp;
+
+struct blockgroup_lock {
+	struct bgl_lock locks[NR_BG_LOCKS];
+};
+
+static inline void bgl_lock_init(struct blockgroup_lock *bgl)
+{
+	int i;
+
+	for (i = 0; i < NR_BG_LOCKS; i++)
+		spin_lock_init(&bgl->locks[i].lock);
+}
+
+/*
+ * The accessor is a macro so we can embed a blockgroup_lock into different
+ * superblock types
+ */
+#define sb_bgl_lock(sb, block_group) \
+	(&(sb)->s_blockgroup_lock.locks[(block_group) & (NR_BG_LOCKS-1)].lock)
+
+