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futex: rely on get_user_pages() for shared futexes
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On the way of getting rid of the mmap_sem requirement for shared futexes,
start by relying on get_user_pages().

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Peter Zijlstra authored and Ingo Molnar committed Sep 30, 2008
1 parent 94aca1d commit 38d47c1
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Showing 2 changed files with 82 additions and 82 deletions.
2 changes: 2 additions & 0 deletions include/linux/futex.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -164,6 +164,8 @@ union futex_key {
} both;
};

#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = NULL } }

#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
extern void exit_pi_state_list(struct task_struct *curr);
Expand Down
162 changes: 80 additions & 82 deletions kernel/futex.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -161,6 +161,45 @@ static inline int match_futex(union futex_key *key1, union futex_key *key2)
&& key1->both.offset == key2->both.offset);
}

/*
* Take a reference to the resource addressed by a key.
* Can be called while holding spinlocks.
*
*/
static void get_futex_key_refs(union futex_key *key)
{
if (!key->both.ptr)
return;

switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
atomic_inc(&key->shared.inode->i_count);
break;
case FUT_OFF_MMSHARED:
atomic_inc(&key->private.mm->mm_count);
break;
}
}

/*
* Drop a reference to the resource addressed by a key.
* The hash bucket spinlock must not be held.
*/
static void drop_futex_key_refs(union futex_key *key)
{
if (!key->both.ptr)
return;

switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
iput(key->shared.inode);
break;
case FUT_OFF_MMSHARED:
mmdrop(key->private.mm);
break;
}
}

/**
* get_futex_key - Get parameters which are the keys for a futex.
* @uaddr: virtual address of the futex
Expand All @@ -184,7 +223,6 @@ static int get_futex_key(u32 __user *uaddr, struct rw_semaphore *fshared,
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma;
struct page *page;
int err;

Expand All @@ -210,98 +248,47 @@ static int get_futex_key(u32 __user *uaddr, struct rw_semaphore *fshared,
key->private.address = address;
return 0;
}
/*
* The futex is hashed differently depending on whether
* it's in a shared or private mapping. So check vma first.
*/
vma = find_extend_vma(mm, address);
if (unlikely(!vma))
return -EFAULT;

/*
* Permissions.
*/
if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
again:
err = get_user_pages(current, mm, address, 1, 0, 0, &page, NULL);
if (err < 0)
return err;

lock_page(page);
if (!page->mapping) {
unlock_page(page);
put_page(page);
goto again;
}

/*
* Private mappings are handled in a simple way.
*
* NOTE: When userspace waits on a MAP_SHARED mapping, even if
* it's a read-only handle, it's expected that futexes attach to
* the object not the particular process. Therefore we use
* VM_MAYSHARE here, not VM_SHARED which is restricted to shared
* mappings of _writable_ handles.
* the object not the particular process.
*/
if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
key->both.offset |= FUT_OFF_MMSHARED; /* reference taken on mm */
if (PageAnon(page)) {
key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
key->private.mm = mm;
key->private.address = address;
return 0;
}

/*
* Linear file mappings are also simple.
*/
key->shared.inode = vma->vm_file->f_path.dentry->d_inode;
key->both.offset |= FUT_OFF_INODE; /* inode-based key. */
if (likely(!(vma->vm_flags & VM_NONLINEAR))) {
key->shared.pgoff = (((address - vma->vm_start) >> PAGE_SHIFT)
+ vma->vm_pgoff);
return 0;
} else {
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
key->shared.inode = page->mapping->host;
key->shared.pgoff = page->index;
}

/*
* We could walk the page table to read the non-linear
* pte, and get the page index without fetching the page
* from swap. But that's a lot of code to duplicate here
* for a rare case, so we simply fetch the page.
*/
err = get_user_pages(current, mm, address, 1, 0, 0, &page, NULL);
if (err >= 0) {
key->shared.pgoff =
page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
put_page(page);
return 0;
}
return err;
}
get_futex_key_refs(key);

/*
* Take a reference to the resource addressed by a key.
* Can be called while holding spinlocks.
*
*/
static void get_futex_key_refs(union futex_key *key)
{
if (key->both.ptr == NULL)
return;
switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
atomic_inc(&key->shared.inode->i_count);
break;
case FUT_OFF_MMSHARED:
atomic_inc(&key->private.mm->mm_count);
break;
}
unlock_page(page);
put_page(page);
return 0;
}

/*
* Drop a reference to the resource addressed by a key.
* The hash bucket spinlock must not be held.
*/
static void drop_futex_key_refs(union futex_key *key)
static inline
void put_futex_key(struct rw_semaphore *fshared, union futex_key *key)
{
if (!key->both.ptr)
return;
switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
iput(key->shared.inode);
break;
case FUT_OFF_MMSHARED:
mmdrop(key->private.mm);
break;
}
drop_futex_key_refs(key);
}

static u32 cmpxchg_futex_value_locked(u32 __user *uaddr, u32 uval, u32 newval)
Expand Down Expand Up @@ -385,6 +372,7 @@ static int refill_pi_state_cache(void)
/* pi_mutex gets initialized later */
pi_state->owner = NULL;
atomic_set(&pi_state->refcount, 1);
pi_state->key = FUTEX_KEY_INIT;

current->pi_state_cache = pi_state;

Expand Down Expand Up @@ -462,7 +450,7 @@ void exit_pi_state_list(struct task_struct *curr)
struct list_head *next, *head = &curr->pi_state_list;
struct futex_pi_state *pi_state;
struct futex_hash_bucket *hb;
union futex_key key;
union futex_key key = FUTEX_KEY_INIT;

if (!futex_cmpxchg_enabled)
return;
Expand Down Expand Up @@ -725,7 +713,7 @@ static int futex_wake(u32 __user *uaddr, struct rw_semaphore *fshared,
struct futex_hash_bucket *hb;
struct futex_q *this, *next;
struct plist_head *head;
union futex_key key;
union futex_key key = FUTEX_KEY_INIT;
int ret;

if (!bitset)
Expand Down Expand Up @@ -760,6 +748,7 @@ static int futex_wake(u32 __user *uaddr, struct rw_semaphore *fshared,

spin_unlock(&hb->lock);
out:
put_futex_key(fshared, &key);
futex_unlock_mm(fshared);
return ret;
}
Expand All @@ -773,7 +762,7 @@ futex_wake_op(u32 __user *uaddr1, struct rw_semaphore *fshared,
u32 __user *uaddr2,
int nr_wake, int nr_wake2, int op)
{
union futex_key key1, key2;
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
struct futex_hash_bucket *hb1, *hb2;
struct plist_head *head;
struct futex_q *this, *next;
Expand Down Expand Up @@ -873,6 +862,8 @@ futex_wake_op(u32 __user *uaddr1, struct rw_semaphore *fshared,
if (hb1 != hb2)
spin_unlock(&hb2->lock);
out:
put_futex_key(fshared, &key2);
put_futex_key(fshared, &key1);
futex_unlock_mm(fshared);

return ret;
Expand All @@ -886,7 +877,7 @@ static int futex_requeue(u32 __user *uaddr1, struct rw_semaphore *fshared,
u32 __user *uaddr2,
int nr_wake, int nr_requeue, u32 *cmpval)
{
union futex_key key1, key2;
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
struct futex_hash_bucket *hb1, *hb2;
struct plist_head *head1;
struct futex_q *this, *next;
Expand Down Expand Up @@ -974,6 +965,8 @@ static int futex_requeue(u32 __user *uaddr1, struct rw_semaphore *fshared,
drop_futex_key_refs(&key1);

out:
put_futex_key(fshared, &key2);
put_futex_key(fshared, &key1);
futex_unlock_mm(fshared);
return ret;
}
Expand Down Expand Up @@ -1220,6 +1213,7 @@ static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
retry:
futex_lock_mm(fshared);

q.key = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr, fshared, &q.key);
if (unlikely(ret != 0))
goto out_release_sem;
Expand Down Expand Up @@ -1360,6 +1354,7 @@ static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
queue_unlock(&q, hb);

out_release_sem:
put_futex_key(fshared, &q.key);
futex_unlock_mm(fshared);
return ret;
}
Expand Down Expand Up @@ -1411,6 +1406,7 @@ static int futex_lock_pi(u32 __user *uaddr, struct rw_semaphore *fshared,
retry:
futex_lock_mm(fshared);

q.key = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr, fshared, &q.key);
if (unlikely(ret != 0))
goto out_release_sem;
Expand Down Expand Up @@ -1625,6 +1621,7 @@ static int futex_lock_pi(u32 __user *uaddr, struct rw_semaphore *fshared,
queue_unlock(&q, hb);

out_release_sem:
put_futex_key(fshared, &q.key);
futex_unlock_mm(fshared);
if (to)
destroy_hrtimer_on_stack(&to->timer);
Expand Down Expand Up @@ -1671,7 +1668,7 @@ static int futex_unlock_pi(u32 __user *uaddr, struct rw_semaphore *fshared)
struct futex_q *this, *next;
u32 uval;
struct plist_head *head;
union futex_key key;
union futex_key key = FUTEX_KEY_INIT;
int ret, attempt = 0;

retry:
Expand Down Expand Up @@ -1744,6 +1741,7 @@ static int futex_unlock_pi(u32 __user *uaddr, struct rw_semaphore *fshared)
out_unlock:
spin_unlock(&hb->lock);
out:
put_futex_key(fshared, &key);
futex_unlock_mm(fshared);

return ret;
Expand Down

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