[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).

arch/i386/kernel/entry.S

@@ -717,6 +717,7 @@ ENTRY(sys_call_table)
 	.long SYMBOL_NAME(sys_fremovexattr)
 	.long SYMBOL_NAME(sys_tkill)
 	.long SYMBOL_NAME(sys_sendfile64)
+	.long SYMBOL_NAME(sys_futex)		/* 240 */
 
 	.rept NR_syscalls-(.-sys_call_table)/4
 		.long SYMBOL_NAME(sys_ni_syscall)

arch/ppc/kernel/misc.S

@@ -1289,6 +1289,7 @@ _GLOBAL(sys_call_table)
 	.long sys_removexattr
 	.long sys_lremovexattr
 	.long sys_fremovexattr	/* 220 */
+	.long sys_futex
 	.rept NR_syscalls-(.-sys_call_table)/4
 		.long sys_ni_syscall
 	.endr

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 */

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> */
 

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 */

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
 

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

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 */

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.

kernel/Makefile

@@ -15,7 +15,7 @@ export-objs = signal.o sys.o kmod.o context.o ksyms.o pm.o exec_domain.o \
 obj-y     = sched.o dma.o fork.o exec_domain.o panic.o printk.o \
 	    module.o exit.o itimer.o info.o time.o softirq.o resource.o \
 	    sysctl.o acct.o capability.o ptrace.o timer.o user.o \
-	    signal.o sys.o kmod.o context.o 
+	    signal.o sys.o kmod.o context.o futex.o
 
 obj-$(CONFIG_UID16) += uid16.o
 obj-$(CONFIG_MODULES) += ksyms.o

kernel/futex.c

@@ -0,0 +1,232 @@
+/*
+ *  Fast Userspace Mutexes (which I call "Futexes!").
+ *  (C) Rusty Russell, IBM 2002
+ *
+ *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
+ *  enough at me, Linus for the original (flawed) idea, Matthew
+ *  Kirkwood for proof-of-concept implementation.
+ *
+ *  "The futexes are also cursed."
+ *  "But they come in a choice of three flavours!"
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+#include <linux/kernel.h>
+#include <linux/spinlock.h>
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/hash.h>
+#include <linux/init.h>
+#include <linux/fs.h>
+#include <linux/futex.h>
+#include <linux/highmem.h>
+#include <asm/atomic.h>
+
+/* These mutexes are a very simple counter: the winner is the one who
+   decrements from 1 to 0.  The counter starts at 1 when the lock is
+   free.  A value other than 0 or 1 means someone may be sleeping.
+   This is simple enough to work on all architectures, but has the
+   problem that if we never "up" the semaphore it could eventually
+   wrap around. */
+
+/* FIXME: This may be way too small. --RR */
+#define FUTEX_HASHBITS 6
+
+/* We use this instead of a normal wait_queue_t, so we can wake only
+   the relevent ones (hashed queues may be shared) */
+struct futex_q {
+	struct list_head list;
+	struct task_struct *task;
+	/* Page struct and offset within it. */
+	struct page *page;
+	unsigned int offset;
+};
+
+/* The key for the hash is the address + index + offset within page */
+static struct list_head futex_queues[1<<FUTEX_HASHBITS];
+static spinlock_t futex_lock = SPIN_LOCK_UNLOCKED;
+
+static inline struct list_head *hash_futex(struct page *page,
+					   unsigned long offset)
+{
+	unsigned long h;
+
+	/* struct page is shared, so we can hash on its address */
+	h = (unsigned long)page + offset;
+	return &futex_queues[hash_long(h, FUTEX_HASHBITS)];
+}
+
+static inline void wake_one_waiter(struct list_head *head,
+				   struct page *page,
+				   unsigned int offset)
+{
+	struct list_head *i;
+
+	spin_lock(&futex_lock);
+	list_for_each(i, head) {
+		struct futex_q *this = list_entry(i, struct futex_q, list);
+
+		if (this->page == page && this->offset == offset) {
+			wake_up_process(this->task);
+			break;
+		}
+	}
+	spin_unlock(&futex_lock);
+}
+
+/* Add at end to avoid starvation */
+static inline void queue_me(struct list_head *head,
+			    struct futex_q *q,
+			    struct page *page,
+			    unsigned int offset)
+{
+	q->task = current;
+	q->page = page;
+	q->offset = offset;
+
+	spin_lock(&futex_lock);
+	list_add_tail(&q->list, head);
+	spin_unlock(&futex_lock);
+}
+
+static inline void unqueue_me(struct futex_q *q)
+{
+	spin_lock(&futex_lock);
+	list_del(&q->list);
+	spin_unlock(&futex_lock);
+}
+
+/* Get kernel address of the user page and pin it. */
+static struct page *pin_page(unsigned long page_start)
+{
+	struct mm_struct *mm = current->mm;
+	struct page *page;
+	int err;
+
+	down_read(&mm->mmap_sem);
+	err = get_user_pages(current, current->mm, page_start,
+			     1 /* one page */,
+			     1 /* writable */,
+			     0 /* don't force */,
+			     &page,
+			     NULL /* don't return vmas */);
+	up_read(&mm->mmap_sem);
+
+	if (err < 0)
+		return ERR_PTR(err);
+	return page;
+}
+
+/* Try to decrement the user count to zero. */
+static int decrement_to_zero(struct page *page, unsigned int offset)
+{
+	atomic_t *count;
+	int ret = 0;
+
+	count = kmap(page) + offset;
+	/* If we take the semaphore from 1 to 0, it's ours.  If it's
+           zero, decrement anyway, to indicate we are waiting.  If
+           it's negative, don't decrement so we don't wrap... */
+	if (atomic_read(count) >= 0 && atomic_dec_and_test(count))
+		ret = 1;
+	kunmap(page);
+	return ret;
+}
+
+/* Simplified from arch/ppc/kernel/semaphore.c: Paul M. is a genius. */
+static int futex_down(struct list_head *head, struct page *page, int offset)
+{
+	int retval = 0;
+	struct futex_q q;
+
+	current->state = TASK_INTERRUPTIBLE;
+	queue_me(head, &q, page, offset);
+
+	while (!decrement_to_zero(page, offset)) {
+		if (signal_pending(current)) {
+			retval = -EINTR;
+			break;
+		}
+		schedule();
+		current->state = TASK_INTERRUPTIBLE;
+	}
+	current->state = TASK_RUNNING;
+	unqueue_me(&q);
+	/* If we were signalled, we might have just been woken: we
+	   must wake another one.  Otherwise we need to wake someone
+	   else (if they are waiting) so they drop the count below 0,
+	   and when we "up" in userspace, we know there is a
+	   waiter. */
+	wake_one_waiter(head, page, offset);
+	return retval;
+}
+
+static int futex_up(struct list_head *head, struct page *page, int offset)
+{
+	atomic_t *count;
+
+	count = kmap(page) + offset;
+	atomic_set(count, 1);
+	smp_wmb();
+	kunmap(page);
+	wake_one_waiter(head, page, offset);
+	return 0;
+}
+
+asmlinkage int sys_futex(void *uaddr, int op)
+{
+	int ret;
+	unsigned long pos_in_page;
+	struct list_head *head;
+	struct page *page;
+
+	pos_in_page = ((unsigned long)uaddr) % PAGE_SIZE;
+
+	/* Must be "naturally" aligned, and not on page boundary. */
+	if ((pos_in_page % __alignof__(atomic_t)) != 0
+	    || pos_in_page + sizeof(atomic_t) > PAGE_SIZE)
+		return -EINVAL;
+
+	/* Simpler if it doesn't vanish underneath us. */
+	page = pin_page((unsigned long)uaddr - pos_in_page);
+	if (IS_ERR(page))
+		return PTR_ERR(page);
+
+	head = hash_futex(page, pos_in_page);
+	switch (op) {
+	case FUTEX_UP:
+		ret = futex_up(head, page, pos_in_page);
+		break;
+	case FUTEX_DOWN:
+		ret = futex_down(head, page, pos_in_page);
+		break;
+	/* Add other lock types here... */
+	default:
+		ret = -EINVAL;
+	}
+	put_page(page);
+
+	return ret;
+}
+
+static int __init init(void)
+{
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(futex_queues); i++)
+		INIT_LIST_HEAD(&futex_queues[i]);
+	return 0;
+}
+__initcall(init);