Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check() usage
From: Miles Lane
Date: Sat May 01 2010 - 13:34:17 EST
On Tue, Apr 20, 2010 at 9:52 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Apr 20, 2010 at 08:45:28AM -0400, Miles Lane wrote:
>> Is there a patch set for 2.6.34-rc5 I can test?
>
> I will be sending a patchset out later today after testing, but
> please see below for a sneak preview collapsed into a single patch.
>
> Thanx, Paul
>
>> On Tue, Apr 20, 2010 at 8:31 AM, Eric Paris <eparis@xxxxxxxxxx> wrote:
>>
>> > On Tue, 2010-04-20 at 16:23 +0800, Lai Jiangshan wrote:
>> >
>> > > [PATCH] RCU: don't turn off lockdep when find suspicious
>> > rcu_dereference_check() usage
>> > >
>> > > When suspicious rcu_dereference_check() usage is detected, lockdep is
>> > still
>> > > available actually, so we should not call debug_locks_off() in
>> > > lockdep_rcu_dereference().
>> > >
>> > > For get rid of too much "suspicious rcu_dereference_check() usage"
>> > > output when the "if(!debug_locks_off())" statement is removed. This patch
>> > uses
>> > > static variable '__warned's for very usage of "rcu_dereference*()".
>> > >
>> > > One variable per usage, so, Now, we can get multiple complaint
>> > > when we detect multiple different suspicious rcu_dereference_check()
>> > usage.
>> > >
>> > > Requested-by: Eric Paris <eparis@xxxxxxxxxx>
>> > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> >
>> > Although mine was a linux-next kernel and it doesn't appear that I have
>> > rcu_dereference_protected() at all, so I dropped that bit of the patch,
>> > it worked great! I got 4 more complaints to harass people with. Feel
>> > free to add my tested by if you care to.
>> >
>> > Tested-by: Eric Paris <eparis@xxxxxxxxxx>
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 07db2fe..ec9ab49 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -190,6 +190,15 @@ static inline int rcu_read_lock_sched_held(void)
>
> #ifdef CONFIG_PROVE_RCU
>
> +#define __do_rcu_dereference_check(c) \
> + do { \
> + static bool __warned; \
> + if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
> + __warned = true; \
> + lockdep_rcu_dereference(__FILE__, __LINE__); \
> + } \
> + } while (0)
> +
> /**
> * rcu_dereference_check - rcu_dereference with debug checking
> * @p: The pointer to read, prior to dereferencing
> @@ -219,8 +228,7 @@ static inline int rcu_read_lock_sched_held(void)
> */
> #define rcu_dereference_check(p, c) \
> ({ \
> - if (debug_lockdep_rcu_enabled() && !(c)) \
> - lockdep_rcu_dereference(__FILE__, __LINE__); \
> + __do_rcu_dereference_check(c); \
> rcu_dereference_raw(p); \
> })
>
> @@ -237,8 +245,7 @@ static inline int rcu_read_lock_sched_held(void)
> */
> #define rcu_dereference_protected(p, c) \
> ({ \
> - if (debug_lockdep_rcu_enabled() && !(c)) \
> - lockdep_rcu_dereference(__FILE__, __LINE__); \
> + __do_rcu_dereference_check(c); \
> (p); \
> })
>
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index da5e139..e5c0244 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
> * No lock is needed, since the task isn't on tasklist yet,
> * so it can't be moved to another cgroup, which means the
> * freezer won't be removed and will be valid during this
> - * function call.
> + * function call. Nevertheless, apply RCU read-side critical
> + * section to suppress RCU lockdep false positives.
> */
> + rcu_read_lock();
> freezer = task_freezer(task);
> + rcu_read_unlock();
>
> /*
> * The root cgroup is non-freezable, so we can skip the
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 2594e1c..03dd1fa 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -3801,8 +3801,6 @@ void lockdep_rcu_dereference(const char *file, const int line)
> {
> struct task_struct *curr = current;
>
> - if (!debug_locks_off())
> - return;
> printk("\n===================================================\n");
> printk( "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
> printk( "---------------------------------------------------\n");
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 6af210a..14c44ec 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -323,6 +323,15 @@ static inline struct task_group *task_group(struct task_struct *p)
> /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
> static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> {
> + /*
> + * Strictly speaking this rcu_read_lock() is not needed since the
> + * task_group is tied to the cgroup, which in turn can never go away
> + * as long as there are tasks attached to it.
> + *
> + * However since task_group() uses task_subsys_state() which is an
> + * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
> + */
> + rcu_read_lock();
> #ifdef CONFIG_FAIR_GROUP_SCHED
> p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
> p->se.parent = task_group(p)->se[cpu];
> @@ -332,6 +341,7 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> p->rt.rt_rq = task_group(p)->rt_rq[cpu];
> p->rt.parent = task_group(p)->rt_se[cpu];
> #endif
> + rcu_read_unlock();
> }
>
> #else
>
Hi Paul.
Has this patch made it into the Linus tree?
Thanks!
Miles
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