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Date: Sat, 1 May 2010 22:00:43 -0400 From: Miles Lane <miles.lane@...il.com> To: paulmck@...ux.vnet.ibm.com Cc: Eric Paris <eparis@...hat.com>, Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check() usage On Sat, May 1, 2010 at 5:55 PM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote: > On Sat, May 01, 2010 at 01:26:15PM -0400, Miles Lane wrote: >> On Tue, Apr 20, 2010 at 9:52 AM, Paul E. McKenney >> <paulmck@...ux.vnet.ibm.com> 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@...hat.com> 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@...hat.com> >> >> > > Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com> >> >> > >> >> > 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@...hat.com> >> > >> > 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! > > Hello, Miles, > > Not yet -- working with Ingo to get a variant of it into -tip on > its way to Linus's tree. The latest patch stack may be found at > http://lkml.org/lkml/2010/4/30/500. > > Thanx, Paul > What is the rationale for defaulting to showing only one RCU splat? That setting seems likely to reduce the rate at which things get cleaned up. Ciao, Miles -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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