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Subject[RFC][PATCH] tracepoints: Free early tracepoints after RCU is initialized

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

When enabling trace events via the kernel command line, I hit this warning:

WARNING: CPU: 0 PID: 13 at kernel/rcu/srcutree.c:236 check_init_srcu_struct+0xe/0x61
Modules linked in:
CPU: 0 PID: 13 Comm: watchdog/0 Not tainted 4.18.0-rc6-test+ #6
Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
RIP: 0010:check_init_srcu_struct+0xe/0x61
Code: 48 c7 c6 ec 8a 65 b4 e8 ff 79 fe ff 48 89 df 31 f6 e8 f2 fa ff ff 5a
5b 41 5c 5d c3 0f 1f 44 00 00 83 3d 68 94 b8 01 01 75 02 <0f> 0b 48 8b 87 f0
0a 00 00 a8 03 74 45 55 48 89 e5 41 55 41 54 4c
RSP: 0000:ffff96eb9ea03e68 EFLAGS: 00010246
RAX: ffff96eb962b5b01 RBX: ffffffffb4a87420 RCX: 0000000000000001
RDX: ffffffffb3107969 RSI: ffff96eb962b5b40 RDI: ffffffffb4a87420
RBP: ffff96eb9ea03eb0 R08: ffffabbd00cd7f48 R09: 0000000000000000
R10: ffff96eb9ea03e68 R11: ffffffffb4a6eec0 R12: ffff96eb962b5b40
R13: ffff96eb9ea03ef8 R14: ffffffffb3107969 R15: ffffffffb3107948
FS: 0000000000000000(0000) GS:ffff96eb9ea00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff96eb13ab2000 CR3: 0000000192a1e001 CR4: 00000000001606f0
Call Trace:
<IRQ>
? __call_srcu+0x2d/0x290
? rcu_process_callbacks+0x26e/0x448
? allocate_probes+0x2b/0x2b
call_srcu+0x13/0x15
rcu_free_old_probes+0x1f/0x21
rcu_process_callbacks+0x2ed/0x448
__do_softirq+0x172/0x336
irq_exit+0x62/0xb2
smp_apic_timer_interrupt+0x161/0x19e
apic_timer_interrupt+0xf/0x20
</IRQ>

The problem is that the enabling of trace events before RCU is set up will
cause SRCU to give this warning. To avoid this, add a list to store probes
that need to be freed till after RCU is initialized, and then free them
then.

Link: http://lkml.kernel.org/r/20180810113554.1df28050@gandalf.local.home

Fixes: e6753f23d961d ("tracepoint: Make rcuidle tracepoint callers use SRCU")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/tracepoint.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 955148d91b74..36635d1d8045 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -53,6 +53,9 @@ static LIST_HEAD(tracepoint_module_list);
*/
static DEFINE_MUTEX(tracepoints_mutex);

+static struct rcu_head *early_probes;
+static bool ok_to_free_tracepoints;
+
/*
* Note about RCU :
* It is used to delay the free of multiple probes array until a quiescent
@@ -80,11 +83,40 @@ static void rcu_free_old_probes(struct rcu_head *head)
call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);
}

+static __init int release_early_probes(void)
+{
+ struct rcu_head *tmp;
+
+ ok_to_free_tracepoints = true;
+
+ while (early_probes) {
+ tmp = early_probes;
+ early_probes = tmp->next;
+ call_rcu_sched(tmp, rcu_free_old_probes);
+ }
+
+ return 0;
+}
+
+/* RCU is initialized at core_initcall */
+postcore_initcall(release_early_probes);
+
static inline void release_probes(struct tracepoint_func *old)
{
if (old) {
struct tp_probes *tp_probes = container_of(old,
struct tp_probes, probes[0]);
+
+ /*
+ * We can't free probes if RCU is not initialized yet.
+ * Postpone the freeing till after RCU is initialized.
+ */
+ if (unlikely(!ok_to_free_tracepoints)) {
+ tp_probes->rcu.next = early_probes;
+ early_probes = &tp_probes->rcu;
+ return;
+ }
+
/*
* Tracepoint probes are protected by both sched RCU and SRCU,
* by calling the SRCU callback in the sched RCU callback we
--
2.13.6
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