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Subject[PATCH 6/7] ring-buffer: do not grab locks in nmi
From: Steven Rostedt <srostedt@redhat.com>

If ftrace_dump_on_oops is set, and an NMI detects a lockup, then it
will need to read from the ring buffer. But the read side of the
ring buffer still takes locks. This patch adds a check on the read
side that if it is in an NMI, then it will disable the ring buffer
and not take any locks.

Reads can still happen on a disabled ring buffer.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 59 ++++++++++++++++++++++++++++++++++++++------
1 files changed, 51 insertions(+), 8 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 969f7cb..589b3ee 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2466,6 +2466,21 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
}
EXPORT_SYMBOL_GPL(ring_buffer_iter_peek);

+static inline int rb_ok_to_lock(void)
+{
+ /*
+ * If an NMI die dumps out the content of the ring buffer
+ * do not grab locks. We also permanently disable the ring
+ * buffer too. A one time deal is all you get from reading
+ * the ring buffer from an NMI.
+ */
+ if (likely(!in_nmi() && !oops_in_progress))
+ return 1;
+
+ tracing_off_permanent();
+ return 0;
+}
+
/**
* ring_buffer_peek - peek at the next event to be read
* @buffer: The ring buffer to read
@@ -2481,14 +2496,20 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
struct ring_buffer_event *event;
unsigned long flags;
+ int dolock;

if (!cpumask_test_cpu(cpu, buffer->cpumask))
return NULL;

+ dolock = rb_ok_to_lock();
again:
- spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ local_irq_save(flags);
+ if (dolock)
+ spin_lock(&cpu_buffer->reader_lock);
event = rb_buffer_peek(buffer, cpu, ts);
- spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ if (dolock)
+ spin_unlock(&cpu_buffer->reader_lock);
+ local_irq_restore(flags);

if (event && event->type_len == RINGBUF_TYPE_PADDING) {
cpu_relax();
@@ -2540,6 +2561,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
struct ring_buffer_per_cpu *cpu_buffer;
struct ring_buffer_event *event = NULL;
unsigned long flags;
+ int dolock;
+
+ dolock = rb_ok_to_lock();

again:
/* might be called in atomic */
@@ -2549,7 +2573,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
goto out;

cpu_buffer = buffer->buffers[cpu];
- spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ local_irq_save(flags);
+ if (dolock)
+ spin_lock(&cpu_buffer->reader_lock);

event = rb_buffer_peek(buffer, cpu, ts);
if (!event)
@@ -2558,7 +2584,9 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
rb_advance_reader(cpu_buffer);

out_unlock:
- spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ if (dolock)
+ spin_unlock(&cpu_buffer->reader_lock);
+ local_irq_restore(flags);

out:
preempt_enable();
@@ -2757,15 +2785,23 @@ int ring_buffer_empty(struct ring_buffer *buffer)
{
struct ring_buffer_per_cpu *cpu_buffer;
unsigned long flags;
+ int dolock;
int cpu;
int ret;

+ dolock = rb_ok_to_lock();
+
/* yes this is racy, but if you don't like the race, lock the buffer */
for_each_buffer_cpu(buffer, cpu) {
cpu_buffer = buffer->buffers[cpu];
- spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ local_irq_save(flags);
+ if (dolock)
+ spin_lock(&cpu_buffer->reader_lock);
ret = rb_per_cpu_empty(cpu_buffer);
- spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ if (dolock)
+ spin_unlock(&cpu_buffer->reader_lock);
+ local_irq_restore(flags);
+
if (!ret)
return 0;
}
@@ -2783,15 +2819,22 @@ int ring_buffer_empty_cpu(struct ring_buffer *buffer, int cpu)
{
struct ring_buffer_per_cpu *cpu_buffer;
unsigned long flags;
+ int dolock;
int ret;

if (!cpumask_test_cpu(cpu, buffer->cpumask))
return 1;

+ dolock = rb_ok_to_lock();
+
cpu_buffer = buffer->buffers[cpu];
- spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ local_irq_save(flags);
+ if (dolock)
+ spin_lock(&cpu_buffer->reader_lock);
ret = rb_per_cpu_empty(cpu_buffer);
- spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ if (dolock)
+ spin_unlock(&cpu_buffer->reader_lock);
+ local_irq_restore(flags);

return ret;
}
--
1.6.3.1
--


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