Re: [PATCH] Re: today's futex changes

Rusty Russell
2003-09-08T10:23:16Z

In message <Pine.LNX.4.44.0309061723160.1470-100000@localhost.localdomain> you write:
> While here, please let's also fix the get_futex_key VM_NONLINEAR
> case, which was returning the 1 from get_user_pages, taken as an
> error by its callers. And save a few bytes and improve debuggability
> by uninlining the top-level futex_wake, futex_requeue, futex_wait.

OK, I've updated my patch on top of this. Mainly cosmetic, please
review.

Name: Minor Tweaks To Jamie Lokier's Futex Patch
Author: Rusty Russell
Status: Booted on 2.6.0-test4-bk9
Depends: Misc/futex-hugh.patch.gz

D: Minor changes to Jamie's excellent futex patch.
D: 1) Remove obsolete comment above hash array decl.
D: 2) Semantics of futex on read-only pages unclear: require write perm.
D: 3) Clarify comment about TASK_INTERRUPTIBLE.
D: 4) Andrew Morton says spurious wakeup is a bug. Catch it.
D: 5) Use Jenkins hash.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .24731-linux-2.6.0-test4-bk9/kernel/futex.c .24731-linux-2.6.0-test4-bk9.updated/kernel/futex.c
--- .24731-linux-2.6.0-test4-bk9/kernel/futex.c 2003-09-08 10:44:26.000000000 +1000
+++ .24731-linux-2.6.0-test4-bk9.updated/kernel/futex.c 2003-09-08 12:01:23.000000000 +1000
@@ -33,7 +33,7 @@
#include <linux/poll.h>
#include <linux/fs.h>
#include <linux/file.h>
-#include <linux/hash.h>
+#include <linux/jhash.h>
#include <linux/init.h>
#include <linux/futex.h>
#include <linux/mount.h>
@@ -44,6 +44,7 @@
/*
* Futexes are matched on equal values of this key.
* The key type depends on whether it's a shared or private mapping.
+ * Don't rearrange without looking at hash_futex().
*/
union futex_key {
struct {
@@ -79,7 +80,6 @@ struct futex_q {
struct file *filp;
};

-/* The key for the hash is the address + index + offset within page */
static struct list_head futex_queues[1<<FUTEX_HASHBITS];
static spinlock_t futex_lock = SPIN_LOCK_UNLOCKED;

@@ -89,11 +89,12 @@ static struct vfsmount *futex_mnt;
/*
* We hash on the keys returned from get_futex_key (see below).
*/
-static inline struct list_head *hash_futex(union futex_key *key)
+static inline struct list_head *hash_futex(const union futex_key *key)
{
- return &futex_queues[hash_long(key->both.word
- + (unsigned long) key->both.ptr
- + key->both.offset, FUTEX_HASHBITS)];
+ u32 hash = jhash2((u32*)&key->both.word,
+ (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
+ key->both.offset);
+ return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
}

/*
@@ -145,17 +146,13 @@ static int get_futex_key(unsigned long u
/*
* Permissions.
*/
- if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
- return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
+ if (unlikely(vma->vm_flags & VM_IO))
+ return -EPERM;
+ if (unlikely(vma->vm_flags & (VM_READ|VM_WRITE)) != (VM_READ|VM_WRITE))
+ return -EACCES;

/*
* 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.
*/
if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
key->private.mm = mm;
@@ -333,7 +330,6 @@ static int futex_wait(unsigned long uadd
union futex_key key;
struct futex_q q;

- try_again:
init_waitqueue_head(&q.waiters);

down_read(&current->mm->mmap_sem);
@@ -367,10 +363,10 @@ static int futex_wait(unsigned long uadd
/*
* There might have been scheduling since the queue_me(), as we
* cannot hold a spinlock across the get_user() in case it
- * faults. So we cannot just set TASK_INTERRUPTIBLE state when
+ * faults, and we cannot just set TASK_INTERRUPTIBLE state when
* queueing ourselves into the futex hash. This code thus has to
- * rely on the futex_wake() code doing a wakeup after removing
- * the waiter from the list.
+ * rely on the futex_wake() code removing us from hash when it
+ * wakes us up.
*/
add_wait_queue(&q.waiters, &wait);
spin_lock(&futex_lock);
@@ -394,26 +390,19 @@ static int futex_wait(unsigned long uadd
* we are the only user of it.
*/

- /*
- * Were we woken or interrupted for a valid reason?
- */
- ret = unqueue_me(&q);
- if (ret == 0)
+ /* If we were woken (and unqueued), we succeeded, whatever. */
+ if (!unqueue_me(&q))
return 0;
if (time == 0)
return -ETIMEDOUT;
if (signal_pending(current))
return -EINTR;

- /*
- * No, it was a spurious wakeup. Try again. Should never happen. :)
- */
- goto try_again;
+ /* A spurious wakeup. Should never happen. */
+ BUG();

out_unqueue:
- /*
- * Were we unqueued anyway?
- */
+ /* If we were woken (and unqueued), we succeeded, whatever. */
if (!unqueue_me(&q))
ret = 0;
out_release_sem:

--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.