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Date: Fri, 4 Sep 2015 16:24:22 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Petr Mladek <pmladek@...e.com>
Cc: Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Jiri Kosina <jkosina@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: Show the real fqs_state
On Fri, Sep 04, 2015 at 02:11:29PM +0200, Petr Mladek wrote:
> The value of "fqs_state" in struct rcu_state is always RCU_GP_IDLE.
>
> The real state is stored in a local variable in rcu_gp_kthread().
> It is modified by rcu_gp_fqs() via parameter and return value.
> But the actual value is never stored to rsp->fqs_state.
>
> The result is that print_one_rcu_state() does not show the real
> state.
>
> This code has been added 3 years ago by the commit 4cdfc175c25c89ee
> ("rcu: Move quiescent-state forcing into kthread"). I guess that it
> was an overlook or optimization.
>
> Anyway, the value seems to be manipulated only by the thread, except
> for shoving the status. I do not see any risk in updating it directly
> in the struct.
>
> Signed-off-by: Petr Mladek <pmladek@...e.com>
Good catch, but how about the following fix instead?
Thanx, Paul
------------------------------------------------------------------------
rcu: Finish folding ->fqs_state into ->gp_state
Commit commit 4cdfc175c25c89ee ("rcu: Move quiescent-state forcing
into kthread") started the process of folding the old ->fqs_state
into ->gp_state, but did not complete it. This situation does not
cause any malfunction, but can result in extremely confusing trace
output. This commit completes this task of eliminating ->fqs_state
in favor of ->gp_state.
Reported-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 69ab7ce2cf7b..04234936d897 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,7 +97,7 @@ struct rcu_state sname##_state = { \
.level = { &sname##_state.node[0] }, \
.rda = &sname##_data, \
.call = cr, \
- .fqs_state = RCU_GP_IDLE, \
+ .gp_state = RCU_GP_IDLE, \
.gpnum = 0UL - 300UL, \
.completed = 0UL - 300UL, \
.orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
@@ -1949,16 +1949,15 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
/*
* Do one round of quiescent-state forcing.
*/
-static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
+static void rcu_gp_fqs(struct rcu_state *rsp)
{
- int fqs_state = fqs_state_in;
bool isidle = false;
unsigned long maxj;
struct rcu_node *rnp = rcu_get_root(rsp);
WRITE_ONCE(rsp->gp_activity, jiffies);
rsp->n_force_qs++;
- if (fqs_state == RCU_SAVE_DYNTICK) {
+ if (rsp->gp_state == RCU_SAVE_DYNTICK) {
/* Collect dyntick-idle snapshots. */
if (is_sysidle_rcu_state(rsp)) {
isidle = true;
@@ -1967,7 +1966,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
force_qs_rnp(rsp, dyntick_save_progress_counter,
&isidle, &maxj);
rcu_sysidle_report_gp(rsp, isidle, maxj);
- fqs_state = RCU_FORCE_QS;
+ rsp->gp_state = RCU_FORCE_QS;
} else {
/* Handle dyntick-idle and offline CPUs. */
isidle = true;
@@ -1981,7 +1980,6 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
READ_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS);
raw_spin_unlock_irq(&rnp->lock);
}
- return fqs_state;
}
/*
@@ -2045,7 +2043,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
/* Declare grace period done. */
WRITE_ONCE(rsp->completed, rsp->gpnum);
trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
- rsp->fqs_state = RCU_GP_IDLE;
+ rsp->gp_state = RCU_GP_IDLE;
rdp = this_cpu_ptr(rsp->rda);
/* Advance CBs to reduce false positives below. */
needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
@@ -2063,7 +2061,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
*/
static int __noreturn rcu_gp_kthread(void *arg)
{
- int fqs_state;
int gf;
unsigned long j;
int ret;
@@ -2095,7 +2092,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
}
/* Handle quiescent-state forcing. */
- fqs_state = RCU_SAVE_DYNTICK;
+ rsp->gp_state = RCU_SAVE_DYNTICK;
j = jiffies_till_first_fqs;
if (j > HZ) {
j = HZ;
@@ -2123,7 +2120,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
trace_rcu_grace_period(rsp->name,
READ_ONCE(rsp->gpnum),
TPS("fqsstart"));
- fqs_state = rcu_gp_fqs(rsp, fqs_state);
+ rcu_gp_fqs(rsp);
trace_rcu_grace_period(rsp->name,
READ_ONCE(rsp->gpnum),
TPS("fqsend"));
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d5f58e717c8b..9faad70a8246 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -417,12 +417,11 @@ struct rcu_data {
struct rcu_state *rsp;
};
-/* Values for fqs_state field in struct rcu_state. */
+/* Values for gp_state field in struct rcu_state. */
#define RCU_GP_IDLE 0 /* No grace period in progress. */
#define RCU_GP_INIT 1 /* Grace period being initialized. */
#define RCU_SAVE_DYNTICK 2 /* Need to scan dyntick state. */
#define RCU_FORCE_QS 3 /* Need to force quiescent state. */
-#define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
/* Values for nocb_defer_wakeup field in struct rcu_data. */
#define RCU_NOGP_WAKE_NOT 0
@@ -474,9 +473,8 @@ struct rcu_state {
/* The following fields are guarded by the root rcu_node's lock. */
- u8 fqs_state ____cacheline_internodealigned_in_smp;
- /* Force QS state. */
- u8 boost; /* Subject to priority boost. */
+ u8 boost ____cacheline_internodealigned_in_smp;
+ /* Subject to priority boost. */
unsigned long gpnum; /* Current gp number. */
unsigned long completed; /* # of last completed gp. */
struct task_struct *gp_kthread; /* Task for grace periods. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 999c3672f990..ef7093cc9b5c 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -268,7 +268,7 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
gpnum = rsp->gpnum;
seq_printf(m, "c=%ld g=%ld s=%d jfq=%ld j=%x ",
ulong2long(rsp->completed), ulong2long(gpnum),
- rsp->fqs_state,
+ rsp->gp_state,
(long)(rsp->jiffies_force_qs - jiffies),
(int)(jiffies & 0xffff));
seq_printf(m, "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
--
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