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Subject[PATCH] tracing: Prevent RCU EQS breakage in preemptirq events
Date
If context tracking is enabled, causing page fault in preemptirq
irq_enable or irq_disable events triggers the following RCU EQS warning.

Reproducer:

// CONFIG_PREEMPTIRQ_EVENTS=y
// CONFIG_CONTEXT_TRACKING=y
// CONFIG_RCU_EQS_DEBUG=y
# echo 1 > events/preemptirq/irq_disable/enable
# echo 1 > options/userstacktrace

WARNING: CPU: 0 PID: 2574 at kernel/rcu/tree.c:262 rcu_dynticks_eqs_exit+0x48/0x50
CPU: 0 PID: 2574 Comm: sh Not tainted 5.3.0-rc1+ #105
RIP: 0010:rcu_dynticks_eqs_exit+0x48/0x50
Call Trace:
rcu_eqs_exit+0x4e/0xd0
rcu_user_exit+0x13/0x20
__context_tracking_exit.part.0+0x74/0x120
context_tracking_exit.part.0+0x28/0x50
context_tracking_exit+0x1d/0x20
do_page_fault+0xab/0x1b0
do_async_page_fault+0x35/0xb0
async_page_fault+0x3e/0x50
RIP: 0010:arch_stack_walk_user+0x8e/0x100
stack_trace_save_user+0x7d/0xa9
trace_buffer_unlock_commit_regs+0x178/0x220
trace_event_buffer_commit+0x6b/0x200
trace_event_raw_event_preemptirq_template+0x7b/0xc0
trace_hardirqs_off_caller+0xb3/0xf0
trace_hardirqs_off_thunk+0x1a/0x20
entry_SYSCALL_64_after_hwframe+0x3e/0xbe

Details of call trace and RCU EQS/Context:

entry_SYSCALL_64_after_hwframe() EQS: IN, CTX: USER
trace_irq_disable_rcuidle()
rcu_irq_enter_irqson()
rcu_dynticks_eqs_exit() EQS: IN => OUT
stack_trace_save_user() EQS: OUT, CTX: USER
page_fault()
do_page_fault()
exception_enter() EQS: OUT, CTX: USER
context_tracking_exit()
rcu_eqs_exit()
rcu_dynticks_eqs_exit() EQS: OUT => OUT? (warning)

trace_irq_disable/enable_rcuidle() are called from user mode in entry
code, and calls rcu_irq_enter_irqson() in __DO_TRACE(). This can cause
the state "RCU ESQ: OUT but CTX: USER", then stack_trace_save_user() can
cause page fault which calls rcu_dynticks_eqs_exit() again leads to hit
the EQS validation warning if CONFIG_RCU_EQS_DEBUG is enabled.

Fix it by calling exception_enter/exit() around
trace_irq_disable/enable_rcuidle() to enter CONTEXT_KERNEL before
tracing code causes page fault. Also makes the timing of state change to
CONTEXT_KERNL earlier to prevent tracing codes from calling
context_tracking_exit() recursively.

Ideally, the problem can be fixed by calling enter_from_user_mode() before
TRACE_IRQS_OFF in entry codes (then we need to tell lockdep that IRQs are
off eariler) and calling prepare_exit_to_usermode() after TRACE_IRQS_ON.
But this patch will be much simpler and limit the most change to tracing
codes.

Fixes: 865e63b04e9b ("tracing: Add back in rcu_irq_enter/exit_irqson() for rcuidle tracepoints")
Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
---
kernel/context_tracking.c | 6 +++++-
kernel/trace/trace_preemptirq.c | 15 +++++++++++++--
2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index be01a4d627c9..860eaf9780e5 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -148,6 +148,11 @@ void __context_tracking_exit(enum ctx_state state)
return;

if (__this_cpu_read(context_tracking.state) == state) {
+ /*
+ * Change state before executing codes which can trigger
+ * page fault leading unnecessary re-entrance.
+ */
+ __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
if (__this_cpu_read(context_tracking.active)) {
/*
* We are going to run code that may use RCU. Inform
@@ -159,7 +164,6 @@ void __context_tracking_exit(enum ctx_state state)
trace_user_exit(0);
}
}
- __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
}
context_tracking_recursion_exit();
}
diff --git a/kernel/trace/trace_preemptirq.c b/kernel/trace/trace_preemptirq.c
index 4d8e99fdbbbe..031b51cb94d0 100644
--- a/kernel/trace/trace_preemptirq.c
+++ b/kernel/trace/trace_preemptirq.c
@@ -10,6 +10,7 @@
#include <linux/module.h>
#include <linux/ftrace.h>
#include <linux/kprobes.h>
+#include <linux/context_tracking.h>
#include "trace.h"

#define CREATE_TRACE_POINTS
@@ -49,9 +50,14 @@ NOKPROBE_SYMBOL(trace_hardirqs_off);

__visible void trace_hardirqs_on_caller(unsigned long caller_addr)
{
+ enum ctx_state prev_state;
+
if (this_cpu_read(tracing_irq_cpu)) {
- if (!in_nmi())
+ if (!in_nmi()) {
+ prev_state = exception_enter();
trace_irq_enable_rcuidle(CALLER_ADDR0, caller_addr);
+ exception_exit(prev_state);
+ }
tracer_hardirqs_on(CALLER_ADDR0, caller_addr);
this_cpu_write(tracing_irq_cpu, 0);
}
@@ -63,11 +69,16 @@ NOKPROBE_SYMBOL(trace_hardirqs_on_caller);

__visible void trace_hardirqs_off_caller(unsigned long caller_addr)
{
+ enum ctx_state prev_state;
+
if (!this_cpu_read(tracing_irq_cpu)) {
this_cpu_write(tracing_irq_cpu, 1);
tracer_hardirqs_off(CALLER_ADDR0, caller_addr);
- if (!in_nmi())
+ if (!in_nmi()) {
+ prev_state = exception_enter();
trace_irq_disable_rcuidle(CALLER_ADDR0, caller_addr);
+ exception_exit(prev_state);
+ }
}

lockdep_hardirqs_off(CALLER_ADDR0);
--
2.21.0
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