Re: [PATCH 0/4] ipc: shm and msg fixes

[Linus Torvalds]

On Sat, Sep 21, 2013 at 11:30 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
>
> IPC uses security_xxx_free() at two levels: for freeing the structure
> (ie: shm_destroy()) and cleaning up upon error when creating the
> structure (ie: newseg()). For both I believe we can actually use RCU.
> What do you think of the below change, it is specific for shm, and we'd
> need an equivalent for msq and sems.

Ugh.

This code already has rcu-delaying, usign the existing "rcu" list
entry. I hate how you add a *new* rcu list entry, and we basically
case two callbacks.

More importantly, it's wrong. You do the call_rcu() unconditionally,
but it might not be the last use! You need to do it with the same
logic ipc_rcu_putref(), namely at the dropping of the last reference.

So how about just making ipc_rcu_putref() have a RCU callback
argument, and make the code look something like

    ipc_rcu_putref(shp, shm_rcu_free);

and then shm_rcu_free() just does

    #define ipc_rcu_to_struct(p)  ((void *)(p+1))

    void shm_rcu_free(struct rcu_head *head)
    {
        struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
        struct shmid_kernel *shp = ipc_rcu_to_struct(p);

        security_shm_free(shp);
        ipc_rcu_free(head);
    }

(that "ipc_rcu_free()" would do the current vfree-vs-kfree, just not
rcu-delayed, so it would look something like

    void ipc_rcu_free(struct rcu_head *head)
    {
        struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
        if (is_vmalloc_addr(p))
            vfree(p);
        else
            kfree(p);
    }

Other users would then just use

    ipc_rcu_putref(shp, ipc_rcu_free);

until they too decide that they want to do something extra at RCU freeing time..

Hmm?

           Linus
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