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SubjectRe: [PATCH] Initial support for struct vfs_cred [0/1]
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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);
}

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