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SubjectRe: [PATCH] sched: missing locking in sched_domains code
On Sun, 27 Apr 2008 23:12:24 +0200 Heiko Carstens <heiko.carstens@de.ibm.com> wrote:

> From: Heiko Carstens <heiko.carstens@de.ibm.com>
>
> Concurrent calls to detach_destroy_domains and arch_init_sched_domains
> were prevented by the old scheduler subsystem cpu hotplug mutex. When
> this got converted to get_online_cpus() the locking got broken.
> Unlike before now several processes can concurrently enter the critical
> sections that were protected by the old lock.
>
> So add a new sched_domains_mutex which protects these sections again.
>
> Cc: Gautham R Shenoy <ego@in.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Paul Jackson <pj@sgi.com>
> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
> include/linux/sched.h | 2 ++
> kernel/cpuset.c | 2 ++
> kernel/sched.c | 11 +++++++++++
> 3 files changed, 15 insertions(+)
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -7807,14 +7807,23 @@ match2:
> unlock_doms_cur();
> }
>
> +/*
> + * Protects against concurrent calls to detach_destroy_domains
> + * and arch_init_sched_domains.
> + */
> +DEFINE_MUTEX(sched_domains_mutex);
> +
> #if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
> int arch_reinit_sched_domains(void)
> {
> + static DEFINE_MUTEX(arch_reinit_sched_domains_mutex);

leftover hunk.

> int err;
>
> get_online_cpus();
> + mutex_lock(&sched_domains_mutex);
> detach_destroy_domains(&cpu_online_map);
> err = arch_init_sched_domains(&cpu_online_map);
> + mutex_unlock(&sched_domains_mutex);
> put_online_cpus();
>
> return err;
> @@ -7932,10 +7941,12 @@ void __init sched_init_smp(void)
> BUG_ON(sched_group_nodes_bycpu == NULL);
> #endif
> get_online_cpus();
> + mutex_lock(&sched_domains_mutex);
> arch_init_sched_domains(&cpu_online_map);
> cpus_andnot(non_isolated_cpus, cpu_possible_map, cpu_isolated_map);
> if (cpus_empty(non_isolated_cpus))
> cpu_set(smp_processor_id(), non_isolated_cpus);
> + mutex_unlock(&sched_domains_mutex);
> put_online_cpus();
> /* XXX: Theoretical race here - CPU may be hotplugged now */
> hotcpu_notifier(update_sched_domains, 0);
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -809,6 +809,8 @@ struct sched_domain {
> #endif
> };
>
> +extern struct mutex sched_domains_mutex;
> +
> extern void partition_sched_domains(int ndoms_new, cpumask_t *doms_new,
> struct sched_domain_attr *dattr_new);
> extern int arch_reinit_sched_domains(void);
> Index: linux-2.6/kernel/cpuset.c
> ===================================================================
> --- linux-2.6.orig/kernel/cpuset.c
> +++ linux-2.6/kernel/cpuset.c
> @@ -684,7 +684,9 @@ restart:
> rebuild:
> /* Have scheduler rebuild sched domains */
> get_online_cpus();
> + mutex_lock(&sched_domains_mutex);
> partition_sched_domains(ndoms, doms, dattr);
> + mutex_unlock(&sched_domains_mutex);
> put_online_cpus();
>

It seems a bit fragile to take this lock in the caller without even adding
a comment at the callee site which documents the new locking rule.

It would be more robust to take the lock within partition_sched_domains().

partition_sched_domains() already covers itself with lock_doms_cur(). Can
we take that in arch_reinit_sched_domains() rather than adding the new lock?



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