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    SubjectRe: [PATCH] Re: today's futex changes
    Date
    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.
    -
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