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[PATCH 5/5] clocksource/drivers/arm_arch_timer: Avoid infinite recursion when ftrace is enabled

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From: Ding Tianhong <dingtianhong@huawei.com>

On platforms with an arch timer erratum workaround, it's possible for
arch_timer_reg_read_stable() to recurse into itself when certain
tracing options are enabled, leading to stack overflows and related
problems.

For example, when PREEMPT_TRACER and FUNCTION_GRAPH_TRACER are
selected, it's possible to trigger this with:

$ mount -t debugfs nodev /sys/kernel/debug/
$ echo function_graph > /sys/kernel/debug/tracing/current_tracer

The problem is that in such cases, preempt_disable() instrumentation
attempts to acquire a timestamp via trace_clock(), resulting in a call
back to arch_timer_reg_read_stable(), and hence recursion.

This patch changes arch_timer_reg_read_stable() to use
preempt_{disable,enable}_notrace(), which avoids this.

This problem is similar to the fixed by upstream commit 96b3d28bf4
("sched/clock: Prevent tracing recursion in sched_clock_cpu()").

Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
---
 arch/arm64/include/asm/arch_timer.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 74d08e4..a652ce0 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -65,13 +65,13 @@ DECLARE_PER_CPU(const struct arch_timer_erratum_workaround *,
 	u64 _val;							\
 	if (needs_unstable_timer_counter_workaround()) {		\
 		const struct arch_timer_erratum_workaround *wa;		\
-		preempt_disable();					\
+		preempt_disable_notrace();				\
 		wa = __this_cpu_read(timer_unstable_counter_workaround); \
 		if (wa && wa->read_##reg)				\
 			_val = wa->read_##reg();			\
 		else							\
 			_val = read_sysreg(reg);			\
-		preempt_enable();					\
+		preempt_enable_notrace();				\
 	} else {							\
 		_val = read_sysreg(reg);				\
 	}								\
-- 
2.7.4

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(none) Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
  Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
    Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-11 20:50 +0200
      Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 23:50 +0200
        Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-12 01:00 +0200
        Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Ingo Molnar <mingo@kernel.org> - 2017-08-12 01:40 +0200
  [PATCH 1/5] clocksource/drivers/arm_arch_timer: Fix mem frame loop initialization Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
    [PATCH 5/5] clocksource/drivers/arm_arch_timer: Avoid infinite recursion when ftrace is enabled Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
    [PATCH 2/5] clocksource/drivers/em_sti: Fix error return codes in em_sti_probe() Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
    [PATCH 3/5] clocksource/drivers/timer-of: Checking for IS_ERR() instead of NULL Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
    [PATCH 4/5] clocksource/drivers/Kconfig: Fix CLKSRC_PISTACHIO dependencies Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 16:20 +0200
  Re: [GIT PULL] clockevents: fixes for 4.13-rc4 Ingo Molnar <mingo@kernel.org> - 2017-08-13 11:50 +0200

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