Re: [PATCH v2] modules: Only return -EEXIST for modules that have finished loading

[Jessica Yu]

+++ Prarit Bhargava [07/05/19 10:54 -0400]:
> Heiko, it would still be good to get a test of this patch from you.  I
> tested this here at Red Hat on some System Z machines.  Without the
> modification made here in v2, the systems failed to boot ~10% of the time.
> After the modification I do not see any boot failures.  I also was
> able to reproduce the boot issue with the acpi_cpufreq driver on a very
> large & fast x86 system which had closer to 100% failure rate without
> the changes in v2.  After the modification in v2 the system has rebooted
> all weekend without any issues.
>
> P.
>
> ---8<---
>
> Microsoft HyperV disables the X86_FEATURE_SMCA bit on AMD systems, and
> linux guests boot with repeated errors:
>
> amd64_edac_mod: Unknown symbol amd_unregister_ecc_decoder (err -2)
> amd64_edac_mod: Unknown symbol amd_register_ecc_decoder (err -2)
> amd64_edac_mod: Unknown symbol amd_report_gart_errors (err -2)
> amd64_edac_mod: Unknown symbol amd_unregister_ecc_decoder (err -2)
> amd64_edac_mod: Unknown symbol amd_register_ecc_decoder (err -2)
> amd64_edac_mod: Unknown symbol amd_report_gart_errors (err -2)
>
> The warnings occur because the module code erroneously returns -EEXIST
> for modules that have failed to load and are in the process of being
> removed from the module list.
>
> module amd64_edac_mod has a dependency on module edac_mce_amd.  Using
> modules.dep, systemd will load edac_mce_amd for every request of
> amd64_edac_mod.  When the edac_mce_amd module loads, the module has
> state MODULE_STATE_UNFORMED and once the module load fails and the state
> becomes MODULE_STATE_GOING.  Another request for edac_mce_amd module
> executes and add_unformed_module() will erroneously return -EEXIST even
> though the previous instance of edac_mce_amd has MODULE_STATE_GOING.
> Upon receiving -EEXIST, systemd attempts to load amd64_edac_mod, which
> fails because of unknown symbols from edac_mce_amd.
>
> add_unformed_module() must wait to return for any case other than
> MODULE_STATE_LIVE to prevent a race between multiple loads of
> dependent modules.
>
> v2: The initial (old->state != MODULE_STATE_LIVE) change exposed an
> additional issue in the code.  wait_event_interruptible() puts each thread
> to sleep until the a module finishes loading an executes the module_wq
> workqueue.  The result is a long delay during the boot.  Switching to
> wait_event_interruptible_timeout() resolves the sleep problem.
>
> Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Cc: David Arcari <darcari@xxxxxxxxxx>

Hi Prarit,

Thanks a lot for the revised patch. I'll queue this up right after the
merge window is over.

Thanks!

Jessica

> ---
> kernel/module.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 1c429d8d2d74..6c868aabaf37 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3568,12 +3568,12 @@ static int add_unformed_module(struct module *mod)
> 	mutex_lock(&module_mutex);
> 	old = find_module_all(mod->name, strlen(mod->name), true);
> 	if (old != NULL) {
> -		if (old->state == MODULE_STATE_COMING
> -		    || old->state == MODULE_STATE_UNFORMED) {
> +		if (old->state != MODULE_STATE_LIVE) {
> 			/* Wait in case it fails to load. */
> 			mutex_unlock(&module_mutex);
> -			err = wait_event_interruptible(module_wq,
> -					       finished_loading(mod->name));
> +			err = wait_event_interruptible_timeout(module_wq,
> +					       finished_loading(mod->name),
> +					       HZ/1000);
> 			if (err)
> 				goto out_unlocked;
> 			goto again;
> --
> 2.18.1