[PATCH v2] zram: do not lookup algorithm in backends table

[Sergey Senozhatsky]

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.

An example of such deadlock:

- 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

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 drivers/block/zram/zcomp.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 052aa3f65514..0916de952e09 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -63,12 +63,6 @@ static int zcomp_strm_init(struct zcomp_strm *zstrm, struct zcomp *comp)
 
 bool zcomp_available_algorithm(const char *comp)
 {
-	int i;
-
-	i = sysfs_match_string(backends, comp);
-	if (i >= 0)
-		return true;
-
 	/*
 	 * Crypto does not ignore a trailing new line symbol,
 	 * so make sure you don't supply a string containing
@@ -217,6 +211,11 @@ struct zcomp *zcomp_create(const char *compress)
 	struct zcomp *comp;
 	int error;
 
+	/*
+	 * Crypto API will execute /sbin/modprobe if the compression module
+	 * is not loaded yet. We must do it here, otherwise we are about to
+	 * call /sbin/modprobe under CPU hot-plug lock.
+	 */
 	if (!zcomp_available_algorithm(compress))
 		return ERR_PTR(-EINVAL);
 
-- 
2.37.0.rc0.104.g0611611a94-goog