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Date: Fri, 20 Mar 2015 14:06:49 +1030
From: Rusty Russell <rusty@...tcorp.com.au>
To: Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
mathieu.desnoyers@...icios.com, oleg@...hat.com,
paulmck@...ux.vnet.ibm.com, torvalds@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, andi@...stfloor.org,
rostedt@...dmis.org, tglx@...utronix.de, peterz@...radead.org
Subject: Re: [PATCH 1/8] module: Sanitize RCU usage and locking
Peter Zijlstra <peterz@...radead.org> writes:
> Currently the RCU usage in module is an inconsistent mess of RCU and
> RCU-sched, this is broken for CONFIG_PREEMPT where synchronize_rcu()
> does not imply synchronize_sched().
>
> Most usage sites use preempt_{dis,en}able() which is RCU-sched, but
> (most of) the modification sites use synchronize_rcu(). With the
> exception of the module bug list, which actually uses RCU.
>
> Convert everything over to RCU-sched.
>
> Furthermore add lockdep asserts to all sites, because its not at all
> clear to me the required locking is observed, esp. on exported
> functions.
>
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Acked-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
I'm happy to take these via my modules-next tree, once you're ready.
If you prefer a different carrier, I'll just Ack.
Cheers,
Rusty.
> ---
> include/linux/module.h | 12 ++++++++++--
> kernel/module.c | 42 ++++++++++++++++++++++++++++++++++--------
> lib/bug.c | 7 +++++--
> 3 files changed, 49 insertions(+), 12 deletions(-)
>
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -415,14 +415,22 @@ struct symsearch {
> bool unused;
> };
>
> -/* Search for an exported symbol by name. */
> +/*
> + * Search for an exported symbol by name.
> + *
> + * Must be called with module_mutex held or preemption disabled.
> + */
> const struct kernel_symbol *find_symbol(const char *name,
> struct module **owner,
> const unsigned long **crc,
> bool gplok,
> bool warn);
>
> -/* Walk the exported symbol table */
> +/*
> + * Walk the exported symbol table
> + *
> + * Must be called with module_mutex held or preemption disabled.
> + */
> bool each_symbol_section(bool (*fn)(const struct symsearch *arr,
> struct module *owner,
> void *data), void *data);
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -106,6 +106,24 @@ static LIST_HEAD(modules);
> struct list_head *kdb_modules = &modules; /* kdb needs the list of modules */
> #endif /* CONFIG_KGDB_KDB */
>
> +static void module_assert_mutex(void)
> +{
> + lockdep_assert_held(&module_mutex);
> +}
> +
> +static void module_assert_mutex_or_preempt(void)
> +{
> +#ifdef CONFIG_LOCKDEP
> + int rcu_held = rcu_read_lock_sched_held();
> + int mutex_held = 1;
> +
> + if (debug_locks)
> + mutex_held = lockdep_is_held(&module_mutex);
> +
> + WARN_ON(!rcu_held && !mutex_held);
> +#endif
> +}
> +
> #ifdef CONFIG_MODULE_SIG
> #ifdef CONFIG_MODULE_SIG_FORCE
> static bool sig_enforce = true;
> @@ -319,6 +337,8 @@ bool each_symbol_section(bool (*fn)(cons
> #endif
> };
>
> + module_assert_mutex_or_preempt();
> +
> if (each_symbol_in_section(arr, ARRAY_SIZE(arr), NULL, fn, data))
> return true;
>
> @@ -458,6 +478,8 @@ static struct module *find_module_all(co
> {
> struct module *mod;
>
> + module_assert_mutex();
> +
> list_for_each_entry(mod, &modules, list) {
> if (!even_unformed && mod->state == MODULE_STATE_UNFORMED)
> continue;
> @@ -1856,8 +1878,8 @@ static void free_module(struct module *m
> list_del_rcu(&mod->list);
> /* Remove this module from bug list, this uses list_del_rcu */
> module_bug_cleanup(mod);
> - /* Wait for RCU synchronizing before releasing mod->list and buglist. */
> - synchronize_rcu();
> + /* Wait for RCU-sched synchronizing before releasing mod->list and buglist. */
> + synchronize_sched();
> mutex_unlock(&module_mutex);
>
> /* This may be NULL, but that's OK */
> @@ -3106,11 +3128,11 @@ static noinline int do_init_module(struc
> mod->init_text_size = 0;
> /*
> * We want to free module_init, but be aware that kallsyms may be
> - * walking this with preempt disabled. In all the failure paths,
> - * we call synchronize_rcu/synchronize_sched, but we don't want
> - * to slow down the success path, so use actual RCU here.
> + * walking this with preempt disabled. In all the failure paths, we
> + * call synchronize_sched, but we don't want to slow down the success
> + * path, so use actual RCU here.
> */
> - call_rcu(&freeinit->rcu, do_free_init);
> + call_rcu_sched(&freeinit->rcu, do_free_init);
> mutex_unlock(&module_mutex);
> wake_up_all(&module_wq);
>
> @@ -3368,8 +3390,8 @@ static int load_module(struct load_info
> /* Unlink carefully: kallsyms could be walking list. */
> list_del_rcu(&mod->list);
> wake_up_all(&module_wq);
> - /* Wait for RCU synchronizing before releasing mod->list. */
> - synchronize_rcu();
> + /* Wait for RCU-sched synchronizing before releasing mod->list. */
> + synchronize_sched();
> mutex_unlock(&module_mutex);
> free_module:
> /* Free lock-classes; relies on the preceding sync_rcu() */
> @@ -3636,6 +3658,8 @@ int module_kallsyms_on_each_symbol(int (
> unsigned int i;
> int ret;
>
> + module_assert_mutex();
> +
> list_for_each_entry(mod, &modules, list) {
> if (mod->state == MODULE_STATE_UNFORMED)
> continue;
> @@ -3810,6 +3834,8 @@ struct module *__module_address(unsigned
> if (addr < module_addr_min || addr > module_addr_max)
> return NULL;
>
> + module_assert_mutex_or_preempt();
> +
> list_for_each_entry_rcu(mod, &modules, list) {
> if (mod->state == MODULE_STATE_UNFORMED)
> continue;
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -66,7 +66,7 @@ static const struct bug_entry *module_fi
> struct module *mod;
> const struct bug_entry *bug = NULL;
>
> - rcu_read_lock();
> + rcu_read_lock_sched();
> list_for_each_entry_rcu(mod, &module_bug_list, bug_list) {
> unsigned i;
>
> @@ -77,7 +77,7 @@ static const struct bug_entry *module_fi
> }
> bug = NULL;
> out:
> - rcu_read_unlock();
> + rcu_read_unlock_sched();
>
> return bug;
> }
> @@ -88,6 +88,8 @@ void module_bug_finalize(const Elf_Ehdr
> char *secstrings;
> unsigned int i;
>
> + lockdep_assert_held(&module_mutex);
> +
> mod->bug_table = NULL;
> mod->num_bugs = 0;
>
> @@ -113,6 +115,7 @@ void module_bug_finalize(const Elf_Ehdr
>
> void module_bug_cleanup(struct module *mod)
> {
> + lockdep_assert_held(&module_mutex);
> list_del_rcu(&mod->bug_list);
> }
>
--
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