[PATCH RESEND] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround
From: mark.rutland@arm.com (Mark Rutland)
Date: 2017-07-10 11:23:31
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On Sun, Jul 09, 2017 at 04:30:54PM +0800, Ding Tianhong wrote:
When enable preempt and debug ftrace, and perform the following steps, the
system will hang:
mount -t debugfs nodev /sys/kernel/debug/
cd /sys/kernel/debug/tracing/
echo function_graph > current_tracer
This is because tracing the preempt_disable/enable calls would cause
trace_clock() which would get local timer to go into infinite recursion
when enable the arch timer erratum workaround for some chips, so Prevent
tracing of preempt_disable/enable() in arch_timer_reg_read_stable().
This problem is similar to the fixed by upstream commit 96b3d28bf4
("sched/clock: Prevent tracing recursion in sched_clock_cpu()").
As I mentioned before, the patch itself looks fine to me, but the commit
message is somewhat difficult to read.
Can we please change this to:
arm64: 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.
With that commit message:
Acked-by: Mark Rutland <mark.rutland@arm.com>
Daniel, Thomas, would you be happy to fold that in when picking this? Or
would you prefer that I fix this up and resend?
Thanks,
Mark.
quoted hunk ↗ jump to hunk
Fixes: 6acc71ccac71 ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs") Signed-off-by: Ding Tianhong <redacted> --- 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..67bb7a4 100644 --- a/arch/arm64/include/asm/arch_timer.h +++ b/arch/arm64/include/asm/arch_timer.h@@ -65,13 +65,13 @@ 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); \ } \-- 1.9.0