Re: [PATCH] zram: do not lookup algorithm in backends table

[Sergey Senozhatsky]

On (22/06/22 11:35), Sergey Senozhatsky wrote:
> Always use crypto_has_comp() so that crypto can lookup module,
> call usermodhelper to load the modules, wait for usermodhelper
> to finish and so on. Otherwise crypto will do all of these steps
> under CPU hot-plug lock and this looks like too much stuff to
> handle under the CPU hot-plug lock. Besides this can end up in
> a deadlock when usermodhelper triggers a code path that attempts
> to lock the CPU hot-plug lock, that zram already holds.

And we think that we (not exactly "we", our partners) actually
see a deadlock. It goes something like this:

- path A. zram grabs CPU hot-plug lock, execs /sbin/modprobe from crypto
  and waits for modprobe to finish

disksize_store
 zcomp_create
  __cpuhp_state_add_instance
   __cpuhp_state_add_instance_cpuslocked
    zcomp_cpu_up_prepare
     crypto_alloc_base
      crypto_alg_mod_lookup
       call_usermodehelper_exec
        wait_for_completion_killable
         do_wait_for_common
          schedule

- path B. async work kthread that brings in scsi device. It wants to
  register CPUHP states at some point, and it needs the CPU hot-plug
  lock for that, which is owned by zram.

async_run_entry_fn
 scsi_probe_and_add_lun
  scsi_mq_alloc_queue
   blk_mq_init_queue
    blk_mq_init_allocated_queue
     blk_mq_realloc_hw_ctxs
      __cpuhp_state_add_instance
       __cpuhp_state_add_instance_cpuslocked
        mutex_lock
         schedule

- path C. modprobe sleeps, waiting for all aync works to finish.

load_module
 do_init_module
  async_synchronize_full
   async_synchronize_cookie_domain
    schedule

And none can make any progress.

So I think we need to move crypto_alg_mod_lookup()->call_usermodehelper_exec()
out of CPU hot-plug lock and pre-load modules in advance, before we grab the
hot-plug lock.