Re: [PATCH] Initial support for struct vfs_cred [0/1]

From: Luca Barbieri (ldb@ldb.ods.org)
Date: Sat Aug 31 2002 - 13:57:14 EST

But aren't credential changes supposed to be infrequent?

If so, isn't it faster to put the fields directly in task_struct, keep a
separate shared structure and copy them on kernel entry?

Here is some pseudocode to better illustrate the idea.

vfs_shared_cred::lock can be removed if it's not necessary to do atomic
modifications of multiple cred fields (otherwise you could copy and then
exchange a pointer).

vfs_shared_cred::tasklist_lock can be replaced with RCU.

struct vfs_cred_groups
{
        int ngroups;
        gid_t groups[];
};

struct vfs_cred
{
        uid_t uid, gid;
        struct vfs_cred_groups* groups;
};

struct vfs_shared_cred
{
        atomic_t count;
        spinlock_t lock;
        spinlock_t tasklist_lock;
        vfs_cred cred;
};

struct task_struct
{
        struct vfs_cred cred;
        struct vfs_cred* shared_cred;
        list_t shared_cred_list;
};

struct thread_info
{
        unsigned propagate_cred;
};

propagate_cred()
{
        current_thread_info()->propagate_cred = 0;

        spin_lock(&current->shared_cred->lock);
        current->cred = current->shared_cred->cred;
        spin_unlock(&current->shared_cred->lock);
}

kernel_entry() /* asm for every architecture :( */
{
        if(unlikely(current_thread_info()->propagate_cred))
                propagate_cred();
}

change_credentials()
{
        list_t list;

        spin_lock(&current->shared_cred->lock);
        frob(current->shared_cred);
        spin_unlock(&current->shared_cred->lock);

        wmb();

        spin_lock(&current->shared_cred->tasklist_lock);
        list_for_each(list, &current->shared_cred_list)
        {
                struct task_struct* task = list_entry(list, struct task_struct, shared_cred_list);
                task->thread_info->propagate_cred = 1;
        }
        spin_unlock(&current->shared_cred->tasklist_lock);
}

clone()
{
        spin_lock(&current->shared_cred->lock);
        new->cred = current->shared_cred->cred); /* not current->cred! */
        spin_unlock(&current->shared_cred->lock);

        if(flags & CLONE_CRED)
        {
                new->shared_cred = get_shared_cred(current->shared_cred);
                spin_lock(&current->shared_cred->tasklist_lock);
                list_add(new, &current->shared_cred_list);
                spin_unlock(&current->shared_cred->tasklist_lock);
        }
}

static void release_task(struct task_struct * p)
{
        spin_lock(&current->shared_cred->tasklist_lock);
        __list_del(current->shared_cred_list);
        spin_unlock(&current->shared_cred->tasklist_lock);

        put_shared_cred(current->shared_cred);
}


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

This archive was generated by hypermail 2b29 : Sat Aug 31 2002 - 22:00:33 EST