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    Subject[PATCH 1/10] BC: introduce atomic_dec_and_lock_irqsave()
    Oleg Nesterov noticed to me that the construction like
    (used in beancounter patches and free_uid()):

    local_irq_save(flags);
    if (atomic_dec_and_lock(&refcnt, &lock))
    ...

    is not that good for preemtible kernels, since with preemption
    spin_lock() can schedule() to reduce latency. However, it won't schedule
    if interrupts are disabled.

    So this patch introduces atomic_dec_and_lock_irqsave() as a logical
    counterpart to atomic_dec_and_lock().

    Signed-Off-By: Pavel Emelianov <xemul@openvz.org>
    Signed-Off-By: Kirill Korotaev <dev@openvz.org>

    ---

    include/linux/spinlock.h | 6 ++++++
    kernel/user.c | 5 +----
    lib/dec_and_lock.c | 19 +++++++++++++++++++
    3 files changed, 26 insertions(+), 4 deletions(-)

    --- ./include/linux/spinlock.h.dlirq 2006-08-28 10:17:35.000000000 +0400
    +++ ./include/linux/spinlock.h 2006-08-28 11:22:37.000000000 +0400
    @@ -266,6 +266,12 @@ extern int _atomic_dec_and_lock(atomic_t
    #define atomic_dec_and_lock(atomic, lock) \
    __cond_lock(lock, _atomic_dec_and_lock(atomic, lock))

    +extern int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
    + unsigned long *flagsp);
    +#define atomic_dec_and_lock_irqsave(atomic, lock, flags) \
    + __cond_lock(lock, \
    + _atomic_dec_and_lock_irqsave(atomic, lock, &flags))
    +
    /**
    * spin_can_lock - would spin_trylock() succeed?
    * @lock: the spinlock in question.
    --- ./kernel/user.c.dlirq 2006-07-10 12:39:20.000000000 +0400
    +++ ./kernel/user.c 2006-08-28 11:08:56.000000000 +0400
    @@ -108,15 +108,12 @@ void free_uid(struct user_struct *up)
    if (!up)
    return;

    - local_irq_save(flags);
    - if (atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
    + if (atomic_dec_and_lock_irqsave(&up->__count, &uidhash_lock, flags)) {
    uid_hash_remove(up);
    spin_unlock_irqrestore(&uidhash_lock, flags);
    key_put(up->uid_keyring);
    key_put(up->session_keyring);
    kmem_cache_free(uid_cachep, up);
    - } else {
    - local_irq_restore(flags);
    }
    }

    --- ./lib/dec_and_lock.c.dlirq 2006-04-21 11:59:36.000000000 +0400
    +++ ./lib/dec_and_lock.c 2006-08-28 11:22:08.000000000 +0400
    @@ -33,3 +33,22 @@ int _atomic_dec_and_lock(atomic_t *atomi
    }

    EXPORT_SYMBOL(_atomic_dec_and_lock);
    +
    +/*
    + * the same, but takes the lock with _irqsave
    + */
    +int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
    + unsigned long *flagsp)
    +{
    +#ifdef CONFIG_SMP
    + if (atomic_add_unless(atomic, -1, 1))
    + return 0;
    +#endif
    + spin_lock_irqsave(lock, *flagsp);
    + if (atomic_dec_and_test(atomic))
    + return 1;
    + spin_unlock_irqrestore(lock, *flagsp);
    + return 0;
    +}
    +
    +EXPORT_SYMBOL(_atomic_dec_and_lock_irqsave);
    -
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