/*
* Copyright © 2008 Ryan Lortie
* Copyright © 2010 Codethink Limited
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library 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
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, see <http://www.gnu.org/licenses/>.
*
* Author: Ryan Lortie <desrt@desrt.ca>
*/
#include "config.h"
#include "gbitlock.h"
#include <glib/gmessages.h>
#include <glib/gatomic.h>
#include <glib/gslist.h>
#include <glib/gthread.h>
#include <glib/gslice.h>
#include "gthreadprivate.h"
#ifdef G_BIT_LOCK_FORCE_FUTEX_EMULATION
#undef HAVE_FUTEX
#endif
#ifndef HAVE_FUTEX
static GMutex g_futex_mutex;
static GSList *g_futex_address_list = NULL;
#endif
#ifdef HAVE_FUTEX
/*
* We have headers for futex(2) on the build machine. This does not
* imply that every system that ever runs the resulting glib will have
* kernel support for futex, but you'd have to have a pretty old
* kernel in order for that not to be the case.
*
* If anyone actually gets bit by this, please file a bug. :)
*/
#include <linux/futex.h>
#include <sys/syscall.h>
#include <unistd.h>
#ifndef FUTEX_WAIT_PRIVATE
#define FUTEX_WAIT_PRIVATE FUTEX_WAIT
#define FUTEX_WAKE_PRIVATE FUTEX_WAKE
#endif
/* < private >
* g_futex_wait:
* @address: a pointer to an integer
* @value: the value that should be at @address
*
* Atomically checks that the value stored at @address is equal to
* @value and then blocks. If the value stored at @address is not
* equal to @value then this function returns immediately.
*
* To unblock, call g_futex_wake() on @address.
*
* This call may spuriously unblock (for example, in response to the
* process receiving a signal) but this is not guaranteed. Unlike the
* Linux system call of a similar name, there is no guarantee that a
* waiting process will unblock due to a g_futex_wake() call in a
* separate process.
*/
static void
g_futex_wait (const volatile gint *address,
gint value)
{
syscall (__NR_futex, address, (gsize) FUTEX_WAIT_PRIVATE, (gsize) value, NULL);
}
/* < private >
* g_futex_wake:
* @address: a pointer to an integer
*
* Nominally, wakes one thread that is blocked in g_futex_wait() on
* @address (if any thread is currently waiting).
*
* As mentioned in the documention for g_futex_wait(), spurious
* wakeups may occur. As such, this call may result in more than one
* thread being woken up.
*/
static void
g_futex_wake (const volatile gint *address)
{
syscall (__NR_futex, address, (gsize) FUTEX_WAKE_PRIVATE, (gsize) 1, NULL);
}
#else
/* emulate futex(2) */
typedef struct
{
const volatile gint *address;
gint ref_count;
GCond wait_queue;
} WaitAddress;
static WaitAddress *
g_futex_find_address (const volatile gint *address)
{
GSList *node;
for (node = g_futex_address_list; node; node = node->next)
{
WaitAddress *waiter = node->data;
if (waiter->address == address)
return waiter;
}
return NULL;
}
static void
g_futex_wait (const volatile gint *address,
gint value)
{
g_mutex_lock (&g_futex_mutex);
if G_LIKELY (g_atomic_int_get (address) == value)
{
WaitAddress *waiter;
if ((waiter = g_futex_find_address (address)) == NULL)
{
waiter = g_slice_new (WaitAddress);
waiter->address = address;
g_cond_init (&waiter->wait_queue);
waiter->ref_count = 0;
g_futex_address_list =
g_slist_prepend (g_futex_address_list, waiter);
}
waiter->ref_count++;
g_cond_wait (&waiter->wait_queue, &g_futex_mutex);
if (!--waiter->ref_count)
{
g_futex_address_list =
g_slist_remove (g_futex_address_list, waiter);
g_cond_clear (&waiter->wait_queue);
g_slice_free (WaitAddress, waiter);
}
}
g_mutex_unlock (&g_futex_mutex);
}
static void
g_futex_wake (const volatile gint *address)
{
WaitAddress *waiter;
/* need to lock here for two reasons:
* 1) need to acquire/release lock to ensure waiter is not in
* the process of registering a wait
* 2) need to -stay- locked until the end to ensure a wake()
* in another thread doesn't cause 'waiter' to stop existing
*/
g_mutex_lock (&g_futex_mutex);
if ((waiter = g_futex_find_address (address)))
g_cond_signal (&waiter->wait_queue);
g_mutex_unlock (&g_futex_mutex);
}
#endif
#define CONTENTION_CLASSES 11
static volatile gint g_bit_lock_contended[CONTENTION_CLASSES];
#if (defined (i386) || defined (__amd64__))
#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
#define USE_ASM_GOTO 1
#endif
#endif
/**
* g_bit_lock:
* @address: a pointer to an integer
* @lock_bit: a bit value between 0 and 31
*
* Sets the indicated @lock_bit in @address. If the bit is already
* set, this call will block until g_bit_unlock() unsets the
* corresponding bit.
*
* Attempting to lock on two different bits within the same integer is
* not supported and will very probably cause deadlocks.
*
* The value of the bit that is set is (1u << @bit). If @bit is not
* between 0 and 31 then the result is undefined.
*
* This function accesses @address atomically. All other accesses to
* @address must be atomic in order for this function to work
* reliably.
*
* Since: 2.24
**/
void
g_bit_lock (volatile gint *address,
gint lock_bit)
{
#ifdef USE_ASM_GOTO
retry:
__asm__ volatile goto ("lock bts %1, (%0)\n"
"jc %l[contended]"
: /* no output */
: "r" (address), "r" (lock_bit)
: "cc", "memory"
: contended);
return;
contended:
{
guint mask = 1u << lock_bit;
guint v;
v = g_atomic_int_get (address);
if (v & mask)
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
g_atomic_int_add (&g_bit_lock_contended[class], +1);
g_futex_wait (address, v);
g_atomic_int_add (&g_bit_lock_contended[class], -1);
}
}
goto retry;
#else
guint mask = 1u << lock_bit;
guint v;
retry:
v = g_atomic_int_or (address, mask);
if (v & mask)
/* already locked */
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
g_atomic_int_add (&g_bit_lock_contended[class], +1);
g_futex_wait (address, v);
g_atomic_int_add (&g_bit_lock_contended[class], -1);
goto retry;
}
#endif
}
/**
* g_bit_trylock:
* @address: a pointer to an integer
* @lock_bit: a bit value between 0 and 31
*
* Sets the indicated @lock_bit in @address, returning %TRUE if
* successful. If the bit is already set, returns %FALSE immediately.
*
* Attempting to lock on two different bits within the same integer is
* not supported.
*
* The value of the bit that is set is (1u << @bit). If @bit is not
* between 0 and 31 then the result is undefined.
*
* This function accesses @address atomically. All other accesses to
* @address must be atomic in order for this function to work
* reliably.
*
* Returns: %TRUE if the lock was acquired
*
* Since: 2.24
**/
gboolean
g_bit_trylock (volatile gint *address,
gint lock_bit)
{
#ifdef USE_ASM_GOTO
gboolean result;
__asm__ volatile ("lock bts %2, (%1)\n"
"setnc %%al\n"
"movzx %%al, %0"
: "=r" (result)
: "r" (address), "r" (lock_bit)
: "cc", "memory");
return result;
#else
guint mask = 1u << lock_bit;
guint v;
v = g_atomic_int_or (address, mask);
return ~v & mask;
#endif
}
/**
* g_bit_unlock:
* @address: a pointer to an integer
* @lock_bit: a bit value between 0 and 31
*
* Clears the indicated @lock_bit in @address. If another thread is
* currently blocked in g_bit_lock() on this same bit then it will be
* woken up.
*
* This function accesses @address atomically. All other accesses to
* @address must be atomic in order for this function to work
* reliably.
*
* Since: 2.24
**/
void
g_bit_unlock (volatile gint *address,
gint lock_bit)
{
#ifdef USE_ASM_GOTO
asm volatile ("lock btr %1, (%0)"
: /* no output */
: "r" (address), "r" (lock_bit)
: "cc", "memory");
#else
guint mask = 1u << lock_bit;
g_atomic_int_and (address, ~mask);
#endif
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
if (g_atomic_int_get (&g_bit_lock_contended[class]))
g_futex_wake (address);
}
}
/* We emulate pointer-sized futex(2) because the kernel API only
* supports integers.
*
* We assume that the 'interesting' part is always the lower order bits.
* This assumption holds because pointer bitlocks are restricted to
* using the low order bits of the pointer as the lock.
*
* On 32 bits, there is nothing to do since the pointer size is equal to
* the integer size. On little endian the lower-order bits don't move,
* so do nothing. Only on 64bit big endian do we need to do a bit of
* pointer arithmetic: the low order bits are shifted by 4 bytes. We
* have a helper function that always does the right thing here.
*
* Since we always consider the low-order bits of the integer value, a
* simple cast from (gsize) to (guint) always takes care of that.
*
* After that, pointer-sized futex becomes as simple as:
*
* g_futex_wait (g_futex_int_address (address), (guint) value);
*
* and
*
* g_futex_wake (g_futex_int_address (int_address));
*/
static const volatile gint *
g_futex_int_address (const volatile void *address)
{
const volatile gint *int_address = address;
/* this implementation makes these (reasonable) assumptions: */
G_STATIC_ASSERT (G_BYTE_ORDER == G_LITTLE_ENDIAN ||
(G_BYTE_ORDER == G_BIG_ENDIAN &&
sizeof (int) == 4 &&
(sizeof (gpointer) == 4 || sizeof (gpointer) == 8)));
#if G_BYTE_ORDER == G_BIG_ENDIAN && GLIB_SIZEOF_VOID_P == 8
int_address++;
#endif
return int_address;
}
/**
* g_pointer_bit_lock:
* @address: (not nullable): a pointer to a #gpointer-sized value
* @lock_bit: a bit value between 0 and 31
*
* This is equivalent to g_bit_lock, but working on pointers (or other
* pointer-sized values).
*
* For portability reasons, you may only lock on the bottom 32 bits of
* the pointer.
*
* Since: 2.30
**/
void
(g_pointer_bit_lock) (volatile void *address,
gint lock_bit)
{
g_return_if_fail (lock_bit < 32);
{
#ifdef USE_ASM_GOTO
retry:
asm volatile goto ("lock bts %1, (%0)\n"
"jc %l[contended]"
: /* no output */
: "r" (address), "r" ((gsize) lock_bit)
: "cc", "memory"
: contended);
return;
contended:
{
volatile gsize *pointer_address = address;
gsize mask = 1u << lock_bit;
gsize v;
v = (gsize) g_atomic_pointer_get (pointer_address);
if (v & mask)
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
g_atomic_int_add (&g_bit_lock_contended[class], +1);
g_futex_wait (g_futex_int_address (address), v);
g_atomic_int_add (&g_bit_lock_contended[class], -1);
}
}
goto retry;
#else
volatile gsize *pointer_address = address;
gsize mask = 1u << lock_bit;
gsize v;
retry:
v = g_atomic_pointer_or (pointer_address, mask);
if (v & mask)
/* already locked */
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
g_atomic_int_add (&g_bit_lock_contended[class], +1);
g_futex_wait (g_futex_int_address (address), (guint) v);
g_atomic_int_add (&g_bit_lock_contended[class], -1);
goto retry;
}
#endif
}
}
/**
* g_pointer_bit_trylock:
* @address: (not nullable): a pointer to a #gpointer-sized value
* @lock_bit: a bit value between 0 and 31
*
* This is equivalent to g_bit_trylock, but working on pointers (or
* other pointer-sized values).
*
* For portability reasons, you may only lock on the bottom 32 bits of
* the pointer.
*
* Returns: %TRUE if the lock was acquired
*
* Since: 2.30
**/
gboolean
(g_pointer_bit_trylock) (volatile void *address,
gint lock_bit)
{
g_return_val_if_fail (lock_bit < 32, FALSE);
{
#ifdef USE_ASM_GOTO
gboolean result;
asm volatile ("lock bts %2, (%1)\n"
"setnc %%al\n"
"movzx %%al, %0"
: "=r" (result)
: "r" (address), "r" ((gsize) lock_bit)
: "cc", "memory");
return result;
#else
volatile gsize *pointer_address = address;
gsize mask = 1u << lock_bit;
gsize v;
g_return_val_if_fail (lock_bit < 32, FALSE);
v = g_atomic_pointer_or (pointer_address, mask);
return ~v & mask;
#endif
}
}
/**
* g_pointer_bit_unlock:
* @address: (not nullable): a pointer to a #gpointer-sized value
* @lock_bit: a bit value between 0 and 31
*
* This is equivalent to g_bit_unlock, but working on pointers (or other
* pointer-sized values).
*
* For portability reasons, you may only lock on the bottom 32 bits of
* the pointer.
*
* Since: 2.30
**/
void
(g_pointer_bit_unlock) (volatile void *address,
gint lock_bit)
{
g_return_if_fail (lock_bit < 32);
{
#ifdef USE_ASM_GOTO
asm volatile ("lock btr %1, (%0)"
: /* no output */
: "r" (address), "r" ((gsize) lock_bit)
: "cc", "memory");
#else
volatile gsize *pointer_address = address;
gsize mask = 1u << lock_bit;
g_atomic_pointer_and (pointer_address, ~mask);
#endif
{
guint class = ((gsize) address) % G_N_ELEMENTS (g_bit_lock_contended);
if (g_atomic_int_get (&g_bit_lock_contended[class]))
g_futex_wake (g_futex_int_address (address));
}
}
}