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Message-Id: <20210414132413.98062-2-frederic@kernel.org>
Date: Wed, 14 Apr 2021 15:24:12 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E . McKenney" <paulmck@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Uladzislau Rezki <urezki@...il.com>,
Boqun Feng <boqun.feng@...il.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Neeraj Upadhyay <neeraju@...eaurora.org>,
Josh Triplett <josh@...htriplett.org>,
Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 1/2] srcu: Fix broken node geometry after early ssp init
An ssp initialized before rcu_init_geometry() will have its snp hierarchy
based on CONFIG_NR_CPUS.
Once rcu_init_geometry() is called, the nodes distribution is shrinked
and optimized toward meeting the actual possible number of CPUs detected
on boot.
Later on, the ssp that was initialized before rcu_init_geometry() is
confused and sometimes refers to its initial CONFIG_NR_CPUS based node
hierarchy, sometimes to the new num_possible_cpus() based one instead.
For example each of its sdp->mynode remain backward and refer to the
early node leaves that may not exist anymore. On the other hand the
srcu_for_each_node_breadth_first() refers to the new node hierarchy.
There are at least two bad possible outcomes to this:
1) a) A callback enqueued early on an sdp is recorded pending on
sdp->mynode->srcu_data_have_cbs in srcu_funnel_gp_start() with
sdp->mynode pointing to a deep leaf (say 3 levels).
b) The grace period ends after rcu_init_geometry() which shrinks the
nodes level to a single one. srcu_gp_end() walks through the new
snp hierarchy without ever reaching the old leaves so the callback
is never executed.
This is easily reproduced on an 8 CPUs machine with
CONFIG_NR_CPUS >= 32 and "rcupdate.rcu_self_test=1". The
srcu_barrier() after early tests verification never completes and
the boot hangs:
[ 5413.141029] INFO: task swapper/0:1 blocked for more than 4915 seconds.
[ 5413.147564] Not tainted 5.12.0-rc4+ #28
[ 5413.151927] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5413.159753] task:swapper/0 state:D stack: 0 pid: 1 ppid: 0 flags:0x00004000
[ 5413.168099] Call Trace:
[ 5413.170555] __schedule+0x36c/0x930
[ 5413.174057] ? wait_for_completion+0x88/0x110
[ 5413.178423] schedule+0x46/0xf0
[ 5413.181575] schedule_timeout+0x284/0x380
[ 5413.185591] ? wait_for_completion+0x88/0x110
[ 5413.189957] ? mark_held_locks+0x61/0x80
[ 5413.193882] ? mark_held_locks+0x61/0x80
[ 5413.197809] ? _raw_spin_unlock_irq+0x24/0x50
[ 5413.202173] ? wait_for_completion+0x88/0x110
[ 5413.206535] wait_for_completion+0xb4/0x110
[ 5413.210724] ? srcu_torture_stats_print+0x110/0x110
[ 5413.215610] srcu_barrier+0x187/0x200
[ 5413.219277] ? rcu_tasks_verify_self_tests+0x50/0x50
[ 5413.224244] ? rdinit_setup+0x2b/0x2b
[ 5413.227907] rcu_verify_early_boot_tests+0x2d/0x40
[ 5413.232700] do_one_initcall+0x63/0x310
[ 5413.236541] ? rdinit_setup+0x2b/0x2b
[ 5413.240207] ? rcu_read_lock_sched_held+0x52/0x80
[ 5413.244912] kernel_init_freeable+0x253/0x28f
[ 5413.249273] ? rest_init+0x250/0x250
[ 5413.252846] kernel_init+0xa/0x110
[ 5413.256257] ret_from_fork+0x22/0x30
2) An ssp that gets initialized before rcu_init_geometry() and used
afterward will always have stale rdp->mynode references, resulting in
callbacks to be missed in srcu_gp_end(), just like in the previous
scenario.
Solve this with calling rcu_init_geometry() whenever an struct srcu_state
happens to be initialized before rcu_init(). This way we make sure the
RCU nodes hierarchy is properly built and distributed before the nodes
of a struct srcu_state are allocated.
Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Uladzislau Rezki <urezki@...il.com>
---
kernel/rcu/rcu.h | 3 +++
kernel/rcu/srcutree.c | 2 ++
kernel/rcu/tree.c | 18 +++++++++++++++++-
3 files changed, 22 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 75ed367d5b60..24db97cbf76b 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -278,6 +278,7 @@ extern void resched_cpu(int cpu);
extern int rcu_num_lvls;
extern int num_rcu_lvl[];
extern int rcu_num_nodes;
+extern bool rcu_geometry_initialized;
static bool rcu_fanout_exact;
static int rcu_fanout_leaf;
@@ -308,6 +309,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt)
}
}
+extern void rcu_init_geometry(void);
+
/* Returns a pointer to the first leaf rcu_node structure. */
#define rcu_first_leaf_node() (rcu_state.level[rcu_num_lvls - 1])
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 108f9ca06047..05ca3c275af1 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -171,6 +171,8 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
ssp->sda = alloc_percpu(struct srcu_data);
if (!ssp->sda)
return -ENOMEM;
+ if (!rcu_geometry_initialized)
+ rcu_init_geometry();
init_srcu_struct_nodes(ssp);
ssp->srcu_gp_seq_needed_exp = 0;
ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 740f5cd34459..b1d6e60e08d1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4556,17 +4556,33 @@ static void __init rcu_init_one(void)
}
}
+bool rcu_geometry_initialized __read_mostly;
+
/*
* Compute the rcu_node tree geometry from kernel parameters. This cannot
* replace the definitions in tree.h because those are needed to size
* the ->node array in the rcu_state structure.
*/
-static void __init rcu_init_geometry(void)
+void rcu_init_geometry(void)
{
ulong d;
int i;
+ static unsigned long old_nr_cpu_ids;
int rcu_capacity[RCU_NUM_LVLS];
+ if (rcu_geometry_initialized) {
+ /*
+ * Arrange for warning if rcu_init_geometry() was called before
+ * setup_nr_cpu_ids(). We may miss cases when
+ * nr_cpus_ids == NR_CPUS but that shouldn't matter too much.
+ */
+ WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids);
+ return;
+ }
+
+ old_nr_cpu_ids = nr_cpu_ids;
+ rcu_geometry_initialized = true;
+
/*
* Initialize any unspecified boot parameters.
* The default values of jiffies_till_first_fqs and
--
2.25.1
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