[PATCH] nvme: avoid flush dependency in delete controller flow

[Paul E. McKenney]

On Mon, Apr 09, 2018 at 05:50:26PM +0300, Nitzan Carmi wrote:
> nvme_delete_ctrl queues a work on a MEM_RECLAIM queue
> (nvme_delete_wq), which eventually calls cleanup_srcu_struct,
> which in turn flushes a delayed work from an !MEM_RECLAIM
> queue. This is unsafe as we might trigger deadlocks under
> severe memory pressure.
> 
> Since we don't ever invoke call_srcu(), it is safe
> to use the _quiesced() version of srcu cleanup, and
> avoid that flush dependency.
> 
> Signed-off-by: Nitzan Carmi <nitzanc at mellanox.com>

Very good, thank you!

I queued this with a few edits to the commit log (please see below),
so please let me know if I messed anything up.

Given that it only happens under high memory pressure, I am assuming
that sending this up the v4.18 merge window (not this one, but the next
one) will work.  Please let me know if this is more urgent than that.
(If it is not more urgent, I would like to allow the extra time for
people to get used to yet another addition to the SRCU API.)

------------------------------------------------------------------------

commit 6fde861519fc6e4be50c130272be556ef5656324
Author: Nitzan Carmi <nitzanc at mellanox.com>
Date:   Mon Apr 9 17:50:26 2018 +0300

    nvme: Avoid flush dependency in delete controller flow
    
    The nvme_delete_ctrl() function queues a work item on a MEM_RECLAIM
    queue (nvme_delete_wq), which eventually calls cleanup_srcu_struct(),
    which in turn flushes a delayed work from an !MEM_RECLAIM queue. This
    is unsafe as we might trigger deadlocks under severe memory pressure.
    
    Since we don't ever invoke call_srcu(), it is safe to use the shiny new
    _quiesced() version of srcu cleanup, thus avoiding that flush dependency.
    This commit makes that change.
    
    Signed-off-by: Nitzan Carmi <nitzanc at mellanox.com>
    Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 7aeca5db7916..40008d06c365 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -351,7 +351,7 @@ static void nvme_free_ns_head(struct kref *ref)
 	nvme_mpath_remove_disk(head);
 	ida_simple_remove(&head->subsys->ns_ida, head->instance);
 	list_del_init(&head->entry);
-	cleanup_srcu_struct(&head->srcu);
+	cleanup_srcu_struct_quiesced(&head->srcu);
 	kfree(head);
 }