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

Rusty Russell
2003-09-09T04:14:12Z

In message <20030908120234.5d05cda9.akpm@osdl.org> you write:
> Rusty Russell <rusty@rustcorp.com.au> wrote:
> >
> > D: 4) Andrew Morton says spurious wakeup is a bug. Catch it.
>
> Yes, but going BUG() is a bit rude. We can detect the error, we can
> recover from it and it doesn't cause any user data corruption or anything.
> A rude printk is all that is needed here.

OK. Changed.

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

Name: Minor Tweaks To Jamie Lokier's Futex Patch
Author: Rusty Russell
Status: Booted on 2.6.0-test5

D: Minor changes to Jamie's excellent futex patch.
D: 1) Remove obsolete comment above hash array decl.
D: 2) Clarify comment about TASK_INTERRUPTIBLE.
D: 3) Andrew Morton says spurious wakeup is a bug. Catch it.
D: 4) Try Jenkins hash.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .9333-linux-2.6.0-test5/kernel/futex.c .9333-linux-2.6.0-test5.updated/kernel/futex.c
--- .9333-linux-2.6.0-test5/kernel/futex.c 2003-09-09 10:35:05.000000000 +1000
+++ .9333-linux-2.6.0-test5.updated/kernel/futex.c 2003-09-09 14:06:06.000000000 +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 members 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 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)];
}

/*
@@ -333,7 +334,6 @@ 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 +367,10 @@ 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 +394,17 @@ 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. */
+ WARN_ON(!signal_pending(current));
+ return -EINTR;

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