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rcu: Improve diagnostics for blocked critical sections in irq

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If an RCU read-side critical section occurs within an interrupt handler
or a softirq handler, it cannot have been preempted.  Therefore, there is
a check in rcu_read_unlock_special() checking for this error.  However,
when this check triggers, it lacks diagnostic information.  This commit
therefore moves rcu_read_unlock()'s lockdep annotation to follow the
call to __rcu_read_unlock() and changes rcu_read_unlock_special()'s
WARN_ON_ONCE() to an lockdep_rcu_suspicious() in order to locate where
the offending RCU read-side critical section began.  In addition, the
value of the ->rcu_read_unlock_special field is printed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2015-01-21 15:26:03 -08:00
commit d24209bb68
3 changed files with 14 additions and 3 deletions
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7
include/linux/lockdep.h
View file

@ -531,8 +531,13 @@ do { \
# define might_lock_read(lock) do { } while (0)
#endif
#ifdef CONFIG_PROVE_RCU
#ifdef CONFIG_LOCKDEP
void lockdep_rcu_suspicious(const char *file, const int line, const char *s);
#else
static inline void
lockdep_rcu_suspicious(const char *file, const int line, const char *s)
{
}
#endif
#endif /* __LINUX_LOCKDEP_H */

2
include/linux/rcupdate.h
View file

@ -942,9 +942,9 @@ static inline void rcu_read_unlock(void)
{
rcu_lockdep_assert(rcu_is_watching(),
"rcu_read_unlock() used illegally while idle");
rcu_lock_release(&rcu_lock_map);
__release(RCU);
__rcu_read_unlock();
rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
}
/**

8
kernel/rcu/tree_plugin.h
View file

@ -334,7 +334,13 @@ void rcu_read_unlock_special(struct task_struct *t)
}
/* Hardware IRQ handlers cannot block, complain if they get here. */
if (WARN_ON_ONCE(in_irq() || in_serving_softirq())) {
if (in_irq() || in_serving_softirq()) {
lockdep_rcu_suspicious(__FILE__, __LINE__,
"rcu_read_unlock() from irq or softirq with blocking in critical section!!!\n");
pr_alert("->rcu_read_unlock_special: %#x (b: %d, nq: %d)\n",
t->rcu_read_unlock_special.s,
t->rcu_read_unlock_special.b.blocked,
t->rcu_read_unlock_special.b.need_qs);
local_irq_restore(flags);
return;
}