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
