On Thu, May 25, 2023 at 02:57:08PM +0200, Thomas Gleixner wrote:
> purge_fragmented_blocks() accesses vmap_block::free and vmap_block::dirty
> lockless for a quick check.
>
> Add the missing READ/WRITE_ONCE() annotations.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
> mm/vmalloc.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2094,9 +2094,9 @@ static bool purge_fragmented_block(struc
> return false;
>
> /* prevent further allocs after releasing lock */
> - vb->free = 0;
> + WRITE_ONCE(vb->free, 0);
> /* prevent purging it again */
> - vb->dirty = VMAP_BBMAP_BITS;
> + WRITE_ONCE(vb->dirty, VMAP_BBMAP_BITS);
> vb->dirty_min = 0;
> vb->dirty_max = VMAP_BBMAP_BITS;
> spin_lock(&vbq->lock);
> @@ -2124,8 +2124,11 @@ static void purge_fragmented_blocks(int
>
> rcu_read_lock();
> list_for_each_entry_rcu(vb, &vbq->free, free_list) {
> - if (vb->free + vb->dirty != VMAP_BBMAP_BITS ||
> - vb->dirty == VMAP_BBMAP_BITS)
> + unsigned long free = READ_ONCE(vb->free);
> + unsigned long dirty = READ_ONCE(vb->dirty);
> +
> + if (free + dirty != VMAP_BBMAP_BITS ||
> + dirty == VMAP_BBMAP_BITS)
> continue;
>
> spin_lock(&vb->lock);
> @@ -2233,7 +2236,7 @@ static void vb_free(unsigned long addr,
> vb->dirty_min = min(vb->dirty_min, offset);
> vb->dirty_max = max(vb->dirty_max, offset + (1UL << order));
>
> - vb->dirty += 1UL << order;
> + WRITE_ONCE(vb->dirty, vb->dirty + (1UL << order));
This is probably a me thing, but I'm a little confused as to why this is
necessary in a code path distinct from the purge stuff, as this will only
prevent the compiler from being 'creative' with ordering here which seems unlikely to be an issue? Or is it a case of belts + braces?
Also wouldn't we require a READ_ONCE() here and below also?
> if (vb->dirty == VMAP_BBMAP_BITS) {
> BUG_ON(vb->free);
> spin_unlock(&vb->lock);
>
>
