The patch titled
futex: runtime enable pi and robust functionality
has been added to the -mm tree. Its filename is
futex-runtime-enable-pi-and-robust-functionality.patch
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------------------------------------------------------
Subject: futex: runtime enable pi and robust functionality
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Not all architectures implement futex_atomic_cmpxchg_inatomic(). The default
implementation returns -ENOSYS, which is currently not handled inside of the
futex guts.
Futex PI calls and robust list exits with a held futex result in an endless
loop in the futex code on architectures which have no support.
Fixing up every place where futex_atomic_cmpxchg_inatomic() is called would
add a fair amount of extra if/else constructs to the already complex code. It
is also not possible to disable the robust feature before user space tries to
register robust lists.
Compile time disabling is not a good idea either, as there are already
architectures with runtime detection of futex_atomic_cmpxchg_inatomic support.
Detect the functionality at runtime instead by calling
cmpxchg_futex_value_locked() with a NULL pointer from the futex initialization
code. This is guaranteed to fail, but the call of
futex_atomic_cmpxchg_inatomic() happens with pagefaults disabled.
On architectures, which use the asm-generic implementation or have a runtime
CPU feature detection, a -ENOSYS return value disables the PI/robust features.
On architectures with a working implementation the call returns -EFAULT and
the PI/robust features are enabled.
The relevant syscalls return -ENOSYS and the robust list exit code is blocked,
when the detection fails.
Fixes http://lkml.org/lkml/2008/2/11/149
Originally reported by: Lennart Buytenhek
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>
Cc: Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>
Cc: Riku Voipio <riku.voipio@xxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/futex.h | 1 +
kernel/futex.c | 38 ++++++++++++++++++++++++++++++++++----
kernel/futex_compat.c | 9 +++++++++
3 files changed, 44 insertions(+), 4 deletions(-)
diff -puN include/linux/futex.h~futex-runtime-enable-pi-and-robust-functionality include/linux/futex.h
--- a/include/linux/futex.h~futex-runtime-enable-pi-and-robust-functionality
+++ a/include/linux/futex.h
@@ -167,6 +167,7 @@ union futex_key {
#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
extern void exit_pi_state_list(struct task_struct *curr);
+extern int futex_cmpxchg_enabled;
#else
static inline void exit_robust_list(struct task_struct *curr)
{
diff -puN kernel/futex.c~futex-runtime-enable-pi-and-robust-functionality kernel/futex.c
--- a/kernel/futex.c~futex-runtime-enable-pi-and-robust-functionality
+++ a/kernel/futex.c
@@ -60,6 +60,8 @@
#include "rtmutex_common.h"
+int __read_mostly futex_cmpxchg_enabled;
+
#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
/*
@@ -469,6 +471,8 @@ void exit_pi_state_list(struct task_stru
struct futex_hash_bucket *hb;
union futex_key key;
+ if (!futex_cmpxchg_enabled)
+ return;
/*
* We are a ZOMBIE and nobody can enqueue itself on
* pi_state_list anymore, but we have to be careful
@@ -1870,6 +1874,8 @@ asmlinkage long
sys_set_robust_list(struct robust_list_head __user *head,
size_t len)
{
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
/*
* The kernel knows only one size for now:
*/
@@ -1894,6 +1900,9 @@ sys_get_robust_list(int pid, struct robu
struct robust_list_head __user *head;
unsigned long ret;
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (!pid)
head = current->robust_list;
else {
@@ -1997,6 +2006,9 @@ void exit_robust_list(struct task_struct
unsigned long futex_offset;
int rc;
+ if (!futex_cmpxchg_enabled)
+ return;
+
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
@@ -2051,7 +2063,7 @@ void exit_robust_list(struct task_struct
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
- int ret;
+ int ret = -ENOSYS;
int cmd = op & FUTEX_CMD_MASK;
struct rw_semaphore *fshared = NULL;
@@ -2083,13 +2095,16 @@ long do_futex(u32 __user *uaddr, int op,
ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
break;
case FUTEX_LOCK_PI:
- ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
+ if (futex_cmpxchg_enabled)
+ ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
break;
case FUTEX_UNLOCK_PI:
- ret = futex_unlock_pi(uaddr, fshared);
+ if (futex_cmpxchg_enabled)
+ ret = futex_unlock_pi(uaddr, fshared);
break;
case FUTEX_TRYLOCK_PI:
- ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
+ if (futex_cmpxchg_enabled)
+ ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
break;
default:
ret = -ENOSYS;
@@ -2145,8 +2160,23 @@ static struct file_system_type futex_fs_
static int __init init(void)
{
+ u32 curval;
int i;
+ /*
+ * This will fail and we want it. Some arch implementations do
+ * runtime detection of the futex_atomic_cmpxchg_inatomic()
+ * functionality. We want to know that before we call in any
+ * of the complex code paths. Also we want to prevent
+ * registration of robust lists in that case. NULL is
+ * guaranteed to fault and we get -EFAULT on functional
+ * implementation, the non functional ones will return
+ * -ENOSYS.
+ */
+ curval = cmpxchg_futex_value_locked(NULL, 0, 0);
+ if (curval == -EFAULT)
+ futex_cmpxchg_enabled = 1;
+
for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock);
spin_lock_init(&futex_queues[i].lock);
diff -puN kernel/futex_compat.c~futex-runtime-enable-pi-and-robust-functionality kernel/futex_compat.c
--- a/kernel/futex_compat.c~futex-runtime-enable-pi-and-robust-functionality
+++ a/kernel/futex_compat.c
@@ -54,6 +54,9 @@ void compat_exit_robust_list(struct task
compat_long_t futex_offset;
int rc;
+ if (!futex_cmpxchg_enabled)
+ return;
+
/*
* Fetch the list head (which was registered earlier, via
* sys_set_robust_list()):
@@ -115,6 +118,9 @@ asmlinkage long
compat_sys_set_robust_list(struct compat_robust_list_head __user *head,
compat_size_t len)
{
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (unlikely(len != sizeof(*head)))
return -EINVAL;
@@ -130,6 +136,9 @@ compat_sys_get_robust_list(int pid, comp
struct compat_robust_list_head __user *head;
unsigned long ret;
+ if (!futex_cmpxchg_enabled)
+ return -ENOSYS;
+
if (!pid)
head = current->compat_robust_list;
else {
_
Patches currently in -mm which might be from tglx@xxxxxxxxxxxxx are
origin.patch
hrtimer-fix-abs-clock-realtimepatch.patch
git-kvm.patch
quirks-set-en-bit-of-msi-mapping-for-devices-onht-based-nvidia-platform.patch
pci-disable-decoding-during-sizing-of-bars.patch
execute-tasklets-in-the-same-order-they-were-queued.patch
git-x86.patch
git-x86-cpa-build-fix.patch
x86-amd-thermal-interrupt-support.patch
x86-remove-pt_regs-arg-from-smp_thermal_interrupt.patch
x86-visws-fix-printk-format-warnings.patch
x86-minor-cleanup-of-comments-in-processorh.patch
documentation-i386-io-apictxt-fix-description.patch
x86_64-set-cfg_size-for-amd-family-10h-in-case-mmconfig-is-used.patch
rtc-cmos-display-hpet-emulation-mode.patch
x86-cast-cmpxchg-and-cmpxchg_local-result-for-386-and-486.patch
xtime_lock-vs-update_process_times.patch
dmi-prevent-linked-list-corruption-resent.patch
x86-fix-clearcopy_user_page-declarations-in-pageh.patch
futex-fix-init-order.patch
futex-runtime-enable-pi-and-robust-functionality.patch
futex-disable-pi-robust-on-archs-w-o-valid-implementation.patch
x86-configurable-dmi-scanning-code.patch
asm-futexh-should-include-linux-uaccessh.patch
-
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