On (22/06/22 12:19), Andrew Morton wrote: > > 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 > > Nope, can't do that. > > > 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 > > The usermode helper is free to do anything it wants, including > operations that take the CPU hotplug lock. Or operations which might > in the future be changed to take that lock. Agreed. > > - 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. > > If the locking is fixed, why is there still a need to preload modules? We "fix" locking by doing initial crypto compression algorithm lookup outside of hot-plug lock (pre-load). Crypto API handles a list of preloaded modules internally. What we do currently, we call crypto_alloc_base() under hot-plug lock, which calls crypto_alg_mod_lookup(), which figures out that crypto modules list does not contain that module yet so then it modprobes it. With this patch we do the first crypto_alg_mod_lookup() outside of hot-plug lock, so that it safely modprobes compression module. Then when we grab the hot-plug lock and setup per-CPU streams, crypto_alloc_base()->crypto_alg_mod_lookup() figures that module is already on the list so no modprobe is needed.
