Re: Backporting "padata: Remove broken queue flushing"

[Daniel Jordan <daniel.m.jordan@oracle.com>]

Hello Ben,

On Tue, May 19, 2020 at 02:53:05PM +0100, Ben Hutchings wrote:
> I noticed that commit 07928d9bfc81 "padata: Remove broken queue
> flushing" has been backported to most stable branches, but commit
> 6fc4dbcf0276 "padata: Replace delayed timer with immediate workqueue in
> padata_reorder" has not.
>
> Is this correct?  What prevents the parallel_data ref-count from
> dropping to 0 while the timer is scheduled?

Doesn't seem like anything does, looking at 4.19.

I can see a race where the timer function uses a parallel_data after free
whether or not the refcount goes to 0.  Don't think it's likely to happen in
practice because of how small the window is between the serial callback
finishing and the timer being deactivated.


   task1:
   padata_reorder
                                      task2:
                                      padata_do_serial
                                        // object arrives in reorder queue
     // sees reorder_objects > 0,
     //   set timer for 1 second
     mod_timer
     return
                                        padata_reorder
                                          // queue serial work, which finishes
                                          //   (now possibly no more objects
                                          //    left)
                                          |
   task1:                                 |
   // pd is freed one of two ways:        |
   //   1) pcrypt is unloaded             |
   //   2) padata_replace triggered       |
   //      from userspace                 | (small window)
                                          |
   task3:                                 |
   padata_reorder_timer                   |
     // uses pd after free                |
                                          |
                                          del_timer  // too late


If I got this right we might want to backport the commit you mentioned to be on
the safe side.