[PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

Fast user-space mutex implementation, allowing user space to do all
of the normal handling, with a minimal fallback to kernel space for
when there is lock contention.

The kernel space implementation does not keep any per-lock data
structures, but instead does a fast hash on the physical page and offset
of the user-space lock when contended.  Thus no build/teardown costs, or
any scalability costs wrt metadata.

Updated syscall numbers for 2.5.6, and changed FUTEX_UP/DOWN definitions
to be more logical for future expansions (eg.  r/w).

7 files changed, 72 insertions, 3 deletions

diff --git a/include/asm-i386/mman.h b/include/asm-i386/mman.h
index f953c436ce51..85566a0029ca 100644
--- a/include/asm-i386/mman.h
+++ b/include/asm-i386/mman.h
@@ -4,6 +4,7 @@
 #define PROT_READ	0x1		/* page can be read */
 #define PROT_WRITE	0x2		/* page can be written */
 #define PROT_EXEC	0x4		/* page can be executed */
+#define PROT_SEM	0x8		/* page may be used for atomic ops */
 #define PROT_NONE	0x0		/* page can not be accessed */
 
 #define MAP_SHARED	0x01		/* Share changes */
diff --git a/include/asm-i386/unistd.h b/include/asm-i386/unistd.h
index d87ebc3ceff6..84af0bcfdfe3 100644
--- a/include/asm-i386/unistd.h
+++ b/include/asm-i386/unistd.h
@@ -244,6 +244,7 @@
 #define __NR_fremovexattr	237
 #define __NR_tkill		238
 #define __NR_sendfile64		239
+#define __NR_futex		240
 
 /* user-visible error numbers are in the range -1 - -124: see <asm-i386/errno.h> */
 
diff --git a/include/asm-ppc/mman.h b/include/asm-ppc/mman.h
index 04206f671b85..eb7aba2be87a 100644
--- a/include/asm-ppc/mman.h
+++ b/include/asm-ppc/mman.h
@@ -7,6 +7,7 @@
 #define PROT_READ	0x1		/* page can be read */
 #define PROT_WRITE	0x2		/* page can be written */
 #define PROT_EXEC	0x4		/* page can be executed */
+#define PROT_SEM	0x8		/* page may be used for atomic ops */
 #define PROT_NONE	0x0		/* page can not be accessed */
 
 #define MAP_SHARED	0x01		/* Share changes */
diff --git a/include/asm-ppc/unistd.h b/include/asm-ppc/unistd.h
index 3e1bd01ed08f..d881d0ed64c7 100644
--- a/include/asm-ppc/unistd.h
+++ b/include/asm-ppc/unistd.h
@@ -228,6 +228,7 @@
 #define __NR_removexattr	218
 #define __NR_lremovexattr	219
 #define __NR_fremovexattr	220
+#define __NR_futex		221
 
 #define __NR(n)	#n
 
diff --git a/include/linux/futex.h b/include/linux/futex.h
new file mode 100644
index 000000000000..5ebcc879b4c0
--- /dev/null
+++ b/include/linux/futex.h
@@ -0,0 +1,8 @@
+#ifndef _LINUX_FUTEX_H
+#define _LINUX_FUTEX_H
+
+/* Second argument to futex syscall */
+#define FUTEX_UP (0)
+#define FUTEX_DOWN (1)
+
+#endif
diff --git a/include/linux/hash.h b/include/linux/hash.h
new file mode 100644
index 000000000000..acf17bb8e7f9
--- /dev/null
+++ b/include/linux/hash.h
@@ -0,0 +1,58 @@
+#ifndef _LINUX_HASH_H
+#define _LINUX_HASH_H
+/* Fast hashing routine for a long.
+   (C) 2002 William Lee Irwin III, IBM */
+
+/*
+ * Knuth recommends primes in approximately golden ratio to the maximum
+ * integer representable by a machine word for multiplicative hashing.
+ * Chuck Lever verified the effectiveness of this technique:
+ * http://www.citi.umich.edu/techreports/reports/citi-tr-00-1.pdf
+ *
+ * These primes are chosen to be bit-sparse, that is operations on
+ * them can use shifts and additions instead of multiplications for
+ * machines where multiplications are slow.
+ */
+#if BITS_PER_LONG == 32
+/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
+#define GOLDEN_RATIO_PRIME 0x9e370001UL
+#elif BITS_PER_LONG == 64
+/*  2^63 + 2^61 - 2^57 + 2^54 - 2^51 - 2^18 + 1 */
+#define GOLDEN_RATIO_PRIME 0x9e37fffffffc0001UL
+#else
+#error Define GOLDEN_RATIO_PRIME for your wordsize.
+#endif
+
+static inline unsigned long hash_long(unsigned long val, unsigned int bits)
+{
+	unsigned long hash = val;
+
+#if BITS_PER_LONG == 64
+	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
+	unsigned long n = hash;
+	n <<= 18;
+	hash -= n;
+	n <<= 33;
+	hash -= n;
+	n <<= 3;
+	hash += n;
+	n <<= 3;
+	hash -= n;
+	n <<= 4;
+	hash += n;
+	n <<= 2;
+	hash += n;
+#else
+	/* On some cpus multiply is faster, on others gcc will do shifts */
+	hash *= GOLDEN_RATIO_PRIME;
+#endif
+
+	/* High bits are more random, so use them. */
+	return hash >> (BITS_PER_LONG - bits);
+}
+	
+static inline unsigned long hash_ptr(void *ptr, unsigned int bits)
+{
+	return hash_long((unsigned long)ptr, bits);
+}
+#endif /* _LINUX_HASH_H */
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ff810df6c8ee..bfe502d98f2a 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -51,8 +51,7 @@ typedef struct zone_struct {
 	/*
 	 * wait_table		-- the array holding the hash table
 	 * wait_table_size	-- the size of the hash table array
-	 * wait_table_shift	-- wait_table_size
-	 * 				== BITS_PER_LONG (1 << wait_table_bits)
+	 * wait_table_bits	-- wait_table_size == (1 << wait_table_bits)
 	 *
 	 * The purpose of all these is to keep track of the people
 	 * waiting for a page to become available and make them
@@ -75,7 +74,7 @@ typedef struct zone_struct {
 	 */
 	wait_queue_head_t	* wait_table;
 	unsigned long		wait_table_size;
-	unsigned long		wait_table_shift;
+	unsigned long		wait_table_bits;
 
 	/*
 	 * Discontig memory support fields.