[PATCH RFC] possible atomicity issue in ptr_ring_resize_multiple_bh_noprof

[Dheeraj Reddy Jonnalagadda <dheeraj.linuxdev@gmail.com>]

Hi Maintainers,

While reviewing the ptr_ring_resize_multiple_bh_noprof function, I noticed 
a potential atomicity issue. The function appears to be callable from multiple
threads, based on the locking patterns and _bh suffix.

The current code frees queues[i] after releasing locks:

    for (i = 0; i < nrings; ++i) {
        spin_lock_bh(&(rings[i])->consumer_lock);
        spin_lock(&(rings[i])->producer_lock);
        queues[i] = __ptr_ring_swap_queue(rings[i], queues[i],
                                         size, gfp, destroy);
        spin_unlock(&(rings[i])->producer_lock);
        spin_unlock_bh(&(rings[i])->consumer_lock);
    }

    /* Free after releasing locks */
    for (i = 0; i < nrings; ++i)
        kvfree(queues[i]);

It seems that there could be a race condition where another thread modifies
queues[i] between the unlock and the kvfree. Would it be safer to do the
kvfree while still holding the locks and removing the kvfree loop later
as shown below?

    for (i = 0; i < nrings; ++i) {
        spin_lock_bh(&(rings[i])->consumer_lock);
        spin_lock(&(rings[i])->producer_lock);
        queues[i] = __ptr_ring_swap_queue(rings[i], queues[i],
                                         size, gfp, destroy);
        kvfree(queues[i]);
        spin_unlock(&(rings[i])->producer_lock);
        spin_unlock_bh(&(rings[i])->consumer_lock);
    }

    kfree(queues);

    return 0;

I've attached a potential fix, but would appreciate confirmation on whether
this is actually an issue that needs addressing.

-Dheeraj