Re: [PATCH v4 05/15] futex2: Implement support for different futex sizes
From: Andrey Semashev
Date: Sun Jun 06 2021 - 19:11:14 EST
On 6/6/21 10:12 PM, Davidlohr Bueso wrote:
On Thu, 03 Jun 2021, Andr� Almeida wrote:
Implement support for 8, 16 and 64 bit futexes, along with the existing
32 bit support. Userspace should use flags to specify in the syscall
the size of the *uaddr they are operating on.
Variable sized futexes are useful for implementing atomic primitives in
userspace in an efficient manner. 64bit sized futexes are also
particularly useful when userspace stores information to be used in an
atomic fashion on the futex value, given more room for flexibility.
Note that at least in the past, Linus has been vehemently against 64-bit
futexes.
Basically this additional data, like for implementing read/write locks,
does not need to be in the futex atomic wait/wake parts. You can instead
split the userspace lock into two adjacent 32-bit words and do 64-bit
atomic ops on it.
If the kernel performs a 32-bit atomic operation on the futex and the
userspace operates on it using 64-bit atomics, I think you are losing
the atomicity guarantee. For example, Intel SDM Volume 3 Section 8.1.2.2
reads:
Software should access semaphores (shared memory used for signalling
between multiple processors) using identical addresses and operand
lengths. For example, if one processor accesses a semaphore using a
word access, other processors should not access the semaphore using a
byte access.
I wouldn't be surprised if other architectures had a similar requirement.
Of course, this is a new interface altogether, so this time it might
be fair game.
Thanks,
Davidlohr
Overlapping futexes are not allowed, so userspace can't wait and wake on
the same memory address if the are using different sizes.
Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxxxxx>
---
include/uapi/linux/futex.h | 3 +
kernel/futex2.c | 124 ++++++++++++++++++++++++-------------
2 files changed, 84 insertions(+), 43 deletions(-)
diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
index 06ea9bdfa69e..5786270b0c75 100644
--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -42,7 +42,10 @@
FUTEX_PRIVATE_FLAG)
/* Size argument to futex2 syscall */
+#define FUTEX_8 0
+#define FUTEX_16 1
#define FUTEX_32 2
+#define FUTEX_64 3
#define FUTEX_SIZE_MASK 0x3
diff --git a/kernel/futex2.c b/kernel/futex2.c
index 012d7f7fc17a..1e97e5f2e793 100644
--- a/kernel/futex2.c
+++ b/kernel/futex2.c
@@ -89,9 +89,11 @@ struct futex_bucket {
#define FUTEXV_WAITER_MASK (FUTEX_SIZE_MASK | FUTEX_SHARED_FLAG)
#define is_object_shared ((futexv->objects[i].flags &
FUTEX_SHARED_FLAG) ? true : false)
+#define object_size (futexv->objects[i].flags & FUTEX_SIZE_MASK)
-#define FUT_OFF_INODE 1 /* We set bit 0 if key has a reference on
inode */
-#define FUT_OFF_MMSHARED 2 /* We set bit 1 if key has a reference on
mm */
+#define FUT_OFF_INODE PAGE_SIZE
+#define FUT_OFF_MMSHARED (PAGE_SIZE << 1)
+#define FUT_OFF_SIZE 1
static struct futex_bucket *futex_table;
static unsigned int futex2_hashsize;
@@ -321,6 +323,7 @@ static int futex_get_shared_key(uintptr_t address,
struct mm_struct *mm,
* @uaddr: futex user address
* @key: data that uniquely identifies a futex
* @shared: is this a shared futex?
+ * @flags: flags for the size
*
* For private futexes, each uaddr will be unique for a given
mm_struct, and it
* won't be freed for the life time of the process. For shared
futexes, check
@@ -330,21 +333,41 @@ static int futex_get_shared_key(uintptr_t
address, struct mm_struct *mm,
*/
static struct futex_bucket *futex_get_bucket(void __user *uaddr,
struct futex_key *key,
- bool shared)
+ bool shared, unsigned int flags)
{
uintptr_t address = (uintptr_t)uaddr;
u32 hash_key;
+ size_t size;
+
+ switch (flags) {
+ case FUTEX_8:
+ size = sizeof(u8);
+ break;
+ case FUTEX_16:
+ size = sizeof(u16);
+ break;
+ case FUTEX_32:
+ size = sizeof(u32);
+ break;
+ case FUTEX_64:
+ size = sizeof(u64);
+ break;
+ default:
+ return ERR_PTR(-EINVAL);
+ }
+
/* Checking if uaddr is valid and accessible */
- if (unlikely(!IS_ALIGNED(address, sizeof(u32))))
+ if (unlikely(!IS_ALIGNED(address, size)))
return ERR_PTR(-EINVAL);
- if (unlikely(!access_ok(uaddr, sizeof(u32))))
+ if (unlikely(!access_ok(uaddr, size)))
return ERR_PTR(-EFAULT);
key->offset = address % PAGE_SIZE;
address -= key->offset;
key->pointer = (u64)address;
key->index = (unsigned long)current->mm;
+ key->offset |= FUT_OFF_SIZE << (size - sizeof(u8));
if (shared)
futex_get_shared_key(address, current->mm, key);
@@ -358,18 +381,39 @@ static struct futex_bucket
*futex_get_bucket(void __user *uaddr,
/**
* futex_get_user - Get the userspace value on this address
- * @uval: variable to store the value
- * @uaddr: userspace address
+ * @uval: variable to store the value
+ * @uaddr: userspace address
+ * @pagefault: true if pagefault should be disabled
+ * @flags: flags for the size
*
* Check the comment at futex_enqueue() for more information.
*/
-static int futex_get_user(u32 *uval, u32 __user *uaddr)
+static int futex_get_user(u64 *uval, void __user *uaddr, unsigned int
flags, bool pagefault)
{
int ret;
- pagefault_disable();
- ret = __get_user(*uval, uaddr);
- pagefault_enable();
+ if (pagefault)
+ pagefault_disable();
+
+ switch (flags) {
+ case FUTEX_8:
+ ret = __get_user(*uval, (u8 __user *)uaddr);
+ break;
+ case FUTEX_16:
+ ret = __get_user(*uval, (u16 __user *)uaddr);
+ break;
+ case FUTEX_32:
+ ret = __get_user(*uval, (u32 __user *)uaddr);
+ break;
+ case FUTEX_64:
+ ret = __get_user(*uval, (u64 __user *)uaddr);
+ break;
+ default:
+ BUG();
+ }
+
+ if (pagefault)
+ pagefault_enable();
return ret;
}
@@ -484,8 +528,8 @@ static int futex_enqueue(struct futex_waiter_head
*futexv, unsigned int nr_futex
int *awakened)
{
int i, ret;
- u32 uval, val;
- u32 __user *uaddr;
+ u64 uval, val;
+ void __user *uaddr;
bool retry = false;
struct futex_bucket *bucket;
@@ -493,13 +537,14 @@ static int futex_enqueue(struct
futex_waiter_head *futexv, unsigned int nr_futex
set_current_state(TASK_INTERRUPTIBLE);
for (i = 0; i < nr_futexes; i++) {
- uaddr = (u32 __user *)futexv->objects[i].uaddr;
- val = (u32)futexv->objects[i].val;
+ uaddr = futexv->objects[i].uaddr;
+ val = (u64)futexv->objects[i].val;
if (is_object_shared && retry) {
struct futex_bucket *tmp =
futex_get_bucket((void __user *)uaddr,
- &futexv->objects[i].key, true);
+ &futexv->objects[i].key, true,
+ object_size);
if (IS_ERR(tmp)) {
__set_current_state(TASK_RUNNING);
futex_dequeue_multiple(futexv, i);
@@ -513,7 +558,7 @@ static int futex_enqueue(struct futex_waiter_head
*futexv, unsigned int nr_futex
bucket_inc_waiters(bucket);
spin_lock(&bucket->lock);
- ret = futex_get_user(&uval, uaddr);
+ ret = futex_get_user(&uval, uaddr, object_size, true);
if (unlikely(ret)) {
spin_unlock(&bucket->lock);
@@ -525,7 +570,7 @@ static int futex_enqueue(struct futex_waiter_head
*futexv, unsigned int nr_futex
if (*awakened >= 0)
return 1;
- if (__get_user(uval, uaddr))
+ if (futex_get_user(&uval, uaddr, object_size, false))
return -EFAULT;
retry = true;
@@ -656,9 +701,6 @@ static long ksys_futex_wait(void __user *uaddr,
u64 val, unsigned int flags,
if (flags & ~FUTEX2_MASK)
return -EINVAL;
- if (size != FUTEX_32)
- return -EINVAL;
-
futexv = &wait_single.futexv;
futexv->task = current;
futexv->hint = false;
@@ -667,12 +709,13 @@ static long ksys_futex_wait(void __user *uaddr,
u64 val, unsigned int flags,
waiter->index = 0;
waiter->val = val;
waiter->uaddr = uaddr;
+ waiter->flags = flags;
memset(&wait_single.waiter.key, 0, sizeof(struct futex_key));
INIT_LIST_HEAD(&waiter->list);
/* Get an unlocked hash bucket */
- waiter->bucket = futex_get_bucket(uaddr, &waiter->key, shared);
+ waiter->bucket = futex_get_bucket(uaddr, &waiter->key, shared,
size);
if (IS_ERR(waiter->bucket))
return PTR_ERR(waiter->bucket);
@@ -728,8 +771,7 @@ static int compat_futex_parse_waitv(struct
futex_waiter_head *futexv,
if (copy_from_user(&waitv, &uwaitv[i], sizeof(waitv)))
return -EFAULT;
- if ((waitv.flags & ~FUTEXV_WAITER_MASK) ||
- (waitv.flags & FUTEX_SIZE_MASK) != FUTEX_32)
+ if (waitv.flags & ~FUTEXV_WAITER_MASK)
return -EINVAL;
futexv->objects[i].key.pointer = 0;
@@ -740,7 +782,7 @@ static int compat_futex_parse_waitv(struct
futex_waiter_head *futexv,
bucket = futex_get_bucket(compat_ptr(waitv.uaddr),
&futexv->objects[i].key,
- is_object_shared);
+ is_object_shared, object_size);
if (IS_ERR(bucket))
return PTR_ERR(bucket);
@@ -805,8 +847,7 @@ static int futex_parse_waitv(struct
futex_waiter_head *futexv,
if (copy_from_user(&waitv, &uwaitv[i], sizeof(waitv)))
return -EFAULT;
- if ((waitv.flags & ~FUTEXV_WAITER_MASK) ||
- (waitv.flags & FUTEX_SIZE_MASK) != FUTEX_32)
+ if (waitv.flags & ~FUTEXV_WAITER_MASK)
return -EINVAL;
futexv->objects[i].key.pointer = 0;
@@ -816,7 +857,7 @@ static int futex_parse_waitv(struct
futex_waiter_head *futexv,
futexv->objects[i].index = i;
bucket = futex_get_bucket(waitv.uaddr, &futexv->objects[i].key,
- is_object_shared);
+ is_object_shared, object_size);
if (IS_ERR(bucket))
return PTR_ERR(bucket);
@@ -947,10 +988,7 @@ SYSCALL_DEFINE3(futex_wake, void __user *, uaddr,
unsigned int, nr_wake,
if (flags & ~FUTEX2_MASK)
return -EINVAL;
- if (size != FUTEX_32)
- return -EINVAL;
-
- bucket = futex_get_bucket(uaddr, &waiter.key, shared);
+ bucket = futex_get_bucket(uaddr, &waiter.key, shared, size);
if (IS_ERR(bucket))
return PTR_ERR(bucket);
@@ -987,28 +1025,30 @@ static inline int __futex_requeue(struct
futex_requeue rq1,
bool retry = false;
struct futex_bucket *b1, *b2;
DEFINE_WAKE_Q(wake_q);
- u32 uval;
+ u64 uval;
int ret;
bool shared1 = (rq1.flags & FUTEX_SHARED_FLAG) ? true : false;
bool shared2 = (rq2.flags & FUTEX_SHARED_FLAG) ? true : false;
+ unsigned int size1 = (rq1.flags & FUTEX_SIZE_MASK);
+ unsigned int size2 = (rq2.flags & FUTEX_SIZE_MASK);
- b1 = futex_get_bucket(rq1.uaddr, &w1.key, shared1);
+ b1 = futex_get_bucket(rq1.uaddr, &w1.key, shared1, size1);
if (IS_ERR(b1))
return PTR_ERR(b1);
- b2 = futex_get_bucket(rq2.uaddr, &w2.key, shared2);
+ b2 = futex_get_bucket(rq2.uaddr, &w2.key, shared2, size2);
if (IS_ERR(b2))
return PTR_ERR(b2);
retry:
if (shared1 && retry) {
- b1 = futex_get_bucket(rq1.uaddr, &w1.key, shared1);
+ b1 = futex_get_bucket(rq1.uaddr, &w1.key, shared1, size1);
if (IS_ERR(b1))
return PTR_ERR(b1);
}
if (shared2 && retry) {
- b2 = futex_get_bucket(rq2.uaddr, &w2.key, shared2);
+ b2 = futex_get_bucket(rq2.uaddr, &w2.key, shared2, size2);
if (IS_ERR(b2))
return PTR_ERR(b2);
}
@@ -1027,11 +1067,11 @@ static inline int __futex_requeue(struct
futex_requeue rq1,
spin_lock_nested(&b1->lock, SINGLE_DEPTH_NESTING);
}
- ret = futex_get_user(&uval, rq1.uaddr);
+ ret = futex_get_user(&uval, rq1.uaddr, size1, true);
if (unlikely(ret)) {
futex_double_unlock(b1, b2);
- if (__get_user(uval, (u32 __user *)rq1.uaddr))
+ if (futex_get_user(&uval, rq1.uaddr, size1, false))
return -EFAULT;
bucket_dec_waiters(b2);
@@ -1088,8 +1128,7 @@ static int compat_futex_parse_requeue(struct
futex_requeue *rq,
if (copy_from_user(&tmp, uaddr, sizeof(tmp)))
return -EFAULT;
- if (tmp.flags & ~FUTEXV_WAITER_MASK ||
- (tmp.flags & FUTEX_SIZE_MASK) != FUTEX_32)
+ if (tmp.flags & ~FUTEXV_WAITER_MASK)
return -EINVAL;
rq->uaddr = compat_ptr(tmp.uaddr);
@@ -1134,8 +1173,7 @@ static int futex_parse_requeue(struct
futex_requeue *rq,
if (copy_from_user(rq, uaddr, sizeof(*rq)))
return -EFAULT;
- if (rq->flags & ~FUTEXV_WAITER_MASK ||
- (rq->flags & FUTEX_SIZE_MASK) != FUTEX_32)
+ if (rq->flags & ~FUTEXV_WAITER_MASK)
return -EINVAL;
return 0;
--
2.31.1