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SubjectRe: [PATCH tip/core/rcu 53/55] rcu: Warn when srcu_read_lock() is used in an extended quiescent state
On Tue, Oct 04, 2011 at 11:03:29PM +0200, Frederic Weisbecker wrote:
> On Tue, Sep 06, 2011 at 11:00:47AM -0700, Paul E. McKenney wrote:
> > Catch SRCU up to the other variants of RCU by making PROVE_RCU
> > complain if either srcu_read_lock() or srcu_read_lock_held() are
> > used from within dyntick-idle mode.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> > include/linux/srcu.h | 25 +++++++++++++++----------
> > 1 files changed, 15 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index 58971e8..fcbaee7 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -28,6 +28,7 @@
> > #define _LINUX_SRCU_H
> >
> > #include <linux/mutex.h>
> > +#include <linux/rcupdate.h>
> >
> > struct srcu_struct_array {
> > int c[2];
> > @@ -60,18 +61,10 @@ int __init_srcu_struct(struct srcu_struct *sp, const char *name,
> > __init_srcu_struct((sp), #sp, &__srcu_key); \
> > })
> >
> > -# define srcu_read_acquire(sp) \
> > - lock_acquire(&(sp)->dep_map, 0, 0, 2, 1, NULL, _THIS_IP_)
> > -# define srcu_read_release(sp) \
> > - lock_release(&(sp)->dep_map, 1, _THIS_IP_)
> > -
> > #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> >
> > int init_srcu_struct(struct srcu_struct *sp);
> >
> > -# define srcu_read_acquire(sp) do { } while (0)
> > -# define srcu_read_release(sp) do { } while (0)
> > -
> > #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> >
> > void cleanup_srcu_struct(struct srcu_struct *sp);
> > @@ -90,11 +83,23 @@ long srcu_batches_completed(struct srcu_struct *sp);
> > * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
> > * this assumes we are in an SRCU read-side critical section unless it can
> > * prove otherwise.
> > + *
> > + * Note that if the CPU is in an extended quiescent state, for example,
> > + * if the CPU is in dyntick-idle mode, then rcu_read_lock_held() returns
> > + * false even if the CPU did an rcu_read_lock(). The reason for this is
> > + * that RCU ignores CPUs that are in extended quiescent states, so such
> > + * a CPU is effectively never in an RCU read-side critical section
> > + * regardless of what RCU primitives it invokes. This state of affairs
> > + * is required -- RCU would otherwise need to periodically wake up
> > + * dyntick-idle CPUs, which would defeat the whole purpose of dyntick-idle
> > + * mode.
> > */
> > static inline int srcu_read_lock_held(struct srcu_struct *sp)
> > {
> > if (debug_locks)
> > return lock_is_held(&sp->dep_map);
> > + if (rcu_check_extended_qs())
> > + return 0;
>
> Just to warn you, While rebasing this, I'm also moving things around:

Thank you for letting me know, should not be a problem.

> if (!debug_lock)
> return 1;
>
> if (rcu_is_cpu_idle())
> return 0;
>
> return lock_is_held(&sp->dep_map);
>
> Otherwise we only do the check if lock debugging is disabled,
> which is not what we want I think.

Would it make sense to use this order?

if (rcu_is_cpu_idle())
return 0;

if (!debug_lock)
return 1;

return lock_is_held(&sp->dep_map);

Given the new approach, rcu_is_cpu_idle() works whether or not debug_lock
is enabled.

Thanx, Paul

> > return 1;
> > }
> >
> > @@ -150,7 +155,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
> > {
> > int retval = __srcu_read_lock(sp);
> >
> > - srcu_read_acquire(sp);
> > + rcu_lock_acquire(&(sp)->dep_map);
> > return retval;
> > }
> >
> > @@ -164,7 +169,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
> > static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
> > __releases(sp)
> > {
> > - srcu_read_release(sp);
> > + rcu_lock_release(&(sp)->dep_map);
> > __srcu_read_unlock(sp, idx);
> > }
> >
> > --
> > 1.7.3.2
> >
> > --
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