Commit adb4f11e authored Aug 10, 2017 by Ding Tianhong Committed by Daniel Lezcano Aug 11, 2017

clocksource/drivers/arm_arch_timer: Avoid infinite recursion when ftrace is enabled



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 96b3d28b
("sched/clock: Prevent tracing recursion in sched_clock_cpu()").

Fixes: 6acc71cc ("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>

parent 599dc457

arch/arm64/include/asm/arch_timer.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -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); \
		} \