Re: [PATCH v4 05/15] futex2: Implement support for different futex sizes

[Andrey Semashev <andrey.semashev@gmail.com>]

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@collabora.com>
>> ---
>> 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
>>