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SubjectRe: 2.6.26-rc4: RIP find_pid_ns+0x6b/0xa0
On Tue, May 27, 2008 at 10:06:11AM -0700, Paul E. McKenney wrote:
> On Tue, May 27, 2008 at 09:11:33AM -0700, Linus Torvalds wrote:
> > On Tue, 27 May 2008, Paul E. McKenney wrote:
> > >
> > > But this will only help until preemptible spinlocks arrive, right?
> >
> > I don't think we will ever have preemptible spinlocks.
> >
> > If you preempt spinlocks, you have serious issues with contention and
> > priority inversion etc, and you basically need to turn them into sleeping
> > mutexes. So now you also need to do interrupts as sleepable threads etc
> > etc.
>
> Indeed, all of these are required in that case.
>
> > And it would break the existing non-preempt RCU usage anyway.
>
> Yes, preemptable spinlocks cannot work without preemptable RCU.
>
> > Yeah, maybe the RT people try to do that, but quite frankly, it is insane.
> > Spinlocks are *different* from sleeping locks, for a damn good reason.
>
> Well, I guess I never claimed to be sane...
>
> Anyway, will look at a preemptable RCU that waits for preempt-disable
> sections of code.

And here is a just-now hacked up patch. Untested, probably fails to compile.
Just kicked off a light test run, will let you know how it goes.

Thanx, Paul

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/rcupreempt.h | 15 ++++++++-
kernel/Kconfig.preempt | 15 +++++++++
kernel/rcupreempt.c | 71 ++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 99 insertions(+), 2 deletions(-)
diff -urpNa -X dontdiff linux-2.6.26-rc3/include/linux/rcupreempt.h linux-2.6.26-rc3-rcu-gcwnp/include/linux/rcupreempt.h
--- linux-2.6.26-rc3/include/linux/rcupreempt.h 2008-05-23 02:26:06.000000000 -0700
+++ linux-2.6.26-rc3-rcu-gcwnp/include/linux/rcupreempt.h 2008-05-27 21:27:35.000000000 -0700
@@ -40,7 +40,20 @@
#include <linux/cpumask.h>
#include <linux/seqlock.h>

-#define rcu_qsctr_inc(cpu)
+struct rcu_dyntick_sched {
+ int qs;
+ int rcu_qs_snap;
+};
+
+DECLARE_PER_CPU(struct rcu_dyntick_sched, rcu_dyntick_sched);
+
+static inline void rcu_qsctr_inc(int cpu)
+{
+ struct rcu_dyntick_sched *rdssp = &per_cpu(rcu_dyntick_sched, cpu);
+
+ rdssp->qs++;
+}
+
#define rcu_bh_qsctr_inc(cpu)
#define call_rcu_bh(head, rcu) call_rcu(head, rcu)

diff -urpNa -X dontdiff linux-2.6.26-rc3/kernel/Kconfig.preempt linux-2.6.26-rc3-rcu-gcwnp/kernel/Kconfig.preempt
--- linux-2.6.26-rc3/kernel/Kconfig.preempt 2008-04-16 19:49:44.000000000 -0700
+++ linux-2.6.26-rc3-rcu-gcwnp/kernel/Kconfig.preempt 2008-05-27 21:27:39.000000000 -0700
@@ -77,3 +77,18 @@ config RCU_TRACE

Say Y here if you want to enable RCU tracing
Say N if you are unsure.
+
+config PREEMPT_RCU_WAIT_PREEMPT_DISABLE
+ bool "Cause preemptible RCU to wait for preempt_disable code"
+ depends on PREEMPT_RCU
+ default y
+ help
+ This option causes preemptible RCU's grace periods to wait
+ on preempt_disable() code sections (such as spinlock critical
+ sections in CONFIG_PREEMPT kernels) as well as for RCU
+ read-side critical sections. This preserves this semantic
+ from Classic RCU. Longer term, explicit RCU read-side critical
+ sections need to be added.
+
+ Say N here if you want strict RCU semantics.
+ Say Y if you are unsure.
diff -urpNa -X dontdiff linux-2.6.26-rc3/kernel/rcupreempt.c linux-2.6.26-rc3-rcu-gcwnp/kernel/rcupreempt.c
--- linux-2.6.26-rc3/kernel/rcupreempt.c 2008-05-23 02:26:07.000000000 -0700
+++ linux-2.6.26-rc3-rcu-gcwnp/kernel/rcupreempt.c 2008-05-27 21:46:51.000000000 -0700
@@ -123,6 +123,12 @@ enum rcu_try_flip_states {
rcu_try_flip_waitzero_state,

/*
+ * Wait here for all CPUs to pass through a quiescent state, but
+ * only if CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE.
+ */
+ rcu_try_flip_waitqs_state,
+
+ /*
* Wait here for each of the other CPUs to execute a memory barrier.
* This is necessary to ensure that these other CPUs really have
* completed executing their RCU read-side critical sections, despite
@@ -131,6 +137,14 @@ enum rcu_try_flip_states {
rcu_try_flip_waitmb_state,
};

+/* Plumb the grace-period state machine based on Kconfig parameters. */
+
+#ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE
+#define rcu_try_flip_waitzero_next_state rcu_try_flip_waitqs_state
+#else /* #ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE */
+#define rcu_try_flip_waitzero_next_state rcu_try_flip_waitmb_state
+#endif /* #else #ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE */
+
struct rcu_ctrlblk {
spinlock_t fliplock; /* Protect state-machine transitions. */
long completed; /* Number of last completed batch. */
@@ -413,6 +427,8 @@ static void __rcu_advance_callbacks(stru
}
}

+DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_dyntick_sched, rcu_dyntick_sched);
+
#ifdef CONFIG_NO_HZ

DEFINE_PER_CPU(long, dynticks_progress_counter) = 1;
@@ -619,6 +635,25 @@ rcu_try_flip_waitmb_needed(int cpu)

#endif /* CONFIG_NO_HZ */

+#ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE
+
+void rcu_try_flip_take_qs_snapshot(void)
+{
+ struct rcu_dyntick_sched *rdssp;
+ int cpu;
+
+ for_each_cpu_mask(cpu, rcu_cpu_online_map) {
+ rdssp = &per_cpu(rcu_dyntick_sched, cpu);
+ rdssp->rcu_qs_snap = rdssp->qs;
+ }
+}
+
+#else /* #ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE */
+
+#define rcu_try_flip_take_qs_snapshot()
+
+#endif /* #else #ifdef CONFIG_PREEMPT_RCU_WAIT_PREEMPT_DISABLE */
+
/*
* Get here when RCU is idle. Decide whether we need to
* move out of idle state, and return non-zero if so.
@@ -662,6 +697,13 @@ rcu_try_flip_idle(void)
dyntick_save_progress_counter(cpu);
}

+ /*
+ * And take quiescent-state snapshot if we are also to wait
+ * on preempt_disable() code sequences.
+ */
+
+ rcu_try_flip_take_qs_snapshot();
+
return 1;
}

@@ -731,6 +773,26 @@ rcu_try_flip_waitzero(void)
return 1;
}

+static int
+rcu_try_flip_waitqs(void)
+{
+ int cpu;
+ struct rcu_dyntick_sched *rdssp;
+
+ /* RCU_TRACE_ME(rcupreempt_trace_try_flip_q1); */
+ for_each_cpu_mask(cpu, rcu_cpu_online_map) {
+ rdssp = &per_cpu(rcu_dyntick_sched, cpu);
+ if (rcu_try_flip_waitack_needed(cpu) &&
+ (rdssp->qs == rdssp->rcu_qs_snap)) {
+ /* RCU_TRACE_ME(rcupreempt_trace_try_flip_qe1); */
+ return 0;
+ }
+ }
+
+ /* RCU_TRACE_ME(rcupreempt_trace_try_flip_q2); */
+ return 1;
+}
+
/*
* Wait for all CPUs to do their end-of-grace-period memory barrier.
* Return 0 once all CPUs have done so.
@@ -775,7 +837,9 @@ static void rcu_try_flip(void)

/*
* Take the next transition(s) through the RCU grace-period
- * flip-counter state machine.
+ * flip-counter state machine. The _next_state transition
+ * is defined by the "plumbing" definitions following the
+ * rcu_try_flip_states enum.
*/

switch (rcu_ctrlblk.rcu_try_flip_state) {
@@ -792,6 +856,11 @@ static void rcu_try_flip(void)
case rcu_try_flip_waitzero_state:
if (rcu_try_flip_waitzero())
rcu_ctrlblk.rcu_try_flip_state =
+ rcu_try_flip_waitzero_next_state;
+ break;
+ case rcu_try_flip_waitqs_state:
+ if (rcu_try_flip_waitqs())
+ rcu_ctrlblk.rcu_try_flip_state =
rcu_try_flip_waitmb_state;
break;
case rcu_try_flip_waitmb_state:

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