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

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Date:   Thu, 9 May 2019 15:53:34 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        David Arcari <darcari@...hat.com>
Subject: Re: [PATCH v2] modules: Only return -EEXIST for modules that have
 finished loading

+++ 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@...hat.com>
>Cc: Jessica Yu <jeyu@...nel.org>
>Cc: Heiko Carstens <heiko.carstens@...ibm.com>
>Cc: David Arcari <darcari@...hat.com>

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
>