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Subject[tip: core/rcu] srcu: Fix broken node geometry after early ssp init
The following commit has been merged into the core/rcu branch of tip:

Commit-ID: b5befe842e6612cf894cf4a199924ee872d8b7d8
Gitweb: https://git.kernel.org/tip/b5befe842e6612cf894cf4a199924ee872d8b7d8
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Sat, 17 Apr 2021 15:16:49 +02:00
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Mon, 10 May 2021 16:03:35 -07:00

srcu: Fix broken node geometry after early ssp init

An srcu_struct structure that is initialized before rcu_init_geometry()
will have its srcu_node hierarchy based on CONFIG_NR_CPUS. Once
rcu_init_geometry() is called, this hierarchy is compressed as needed
for the actual maximum number of CPUs for this system.

Later on, that srcu_struct structure is confused, sometimes referring
to its initial CONFIG_NR_CPUS-based hierarchy, and sometimes instead
to the new num_possible_cpus() hierarchy. For example, each of its
->mynode fields continues to reference the original leaf rcu_node
structures, some of which might no longer exist. On the other hand,
srcu_for_each_node_breadth_first() traverses to the new node hierarchy.

There are at least two bad possible outcomes to this:

1) a) A callback enqueued early on an srcu_data structure (call it
*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() shrinks the
nodes level to a single one. srcu_gp_end() walks through the new
srcu_node 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 srcu_struct structure that is 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.

This commit therefore causes init_srcu_struct_nodes to initialize the
geometry, if needed. This ensures that the srcu_node hierarchy is
properly built and distributed from the get-go.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/rcu.h | 2 ++
kernel/rcu/srcutree.c | 3 +++
kernel/rcu/tree.c | 16 +++++++++++++++-
3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index ca3f2af..28de768 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -308,6 +308,8 @@ 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 9ec35c1..5fdbde6 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -90,6 +90,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp)
struct srcu_node *snp;
struct srcu_node *snp_first;

+ /* Initialize geometry if it has not already been initialized. */
+ rcu_init_geometry();
+
/* Work out the overall tree geometry. */
ssp->level[0] = &ssp->node[0];
for (i = 1; i < rcu_num_lvls; i++)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c35b152..0420b23 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4582,11 +4582,25 @@ static void __init rcu_init_one(void)
* 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];
+ static bool initialized;
+
+ if (initialized) {
+ /*
+ * Warn if setup_nr_cpu_ids() had not yet been invoked,
+ * unless nr_cpus_ids == NR_CPUS, in which case who cares?
+ */
+ WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids);
+ return;
+ }
+
+ old_nr_cpu_ids = nr_cpu_ids;
+ initialized = true;

/*
* Initialize any unspecified boot parameters.
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