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Subject[PATCH] tracing: use raw spinlocks instead of spinlocks
When I tried to figure out why my experimental function's return tracer was hanging, I discovered
that it was partially caused by the fact that the ring buffer might use the usual spinlocks during
entry insertion.

ring_buffer_lock_reserve() -> rb_reserve_next_event() -> __rb_reserve_next() -> spin_lock_irqsave()

Since this last function is traced, the result is a recursion during the trace. I guess it
happens too with the function tracer.

We should use the raw_spin_locks which are not traced.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
kernel/trace/ring_buffer.c | 33 ++++++++++++++++++++-------------
1 files changed, 20 insertions(+), 13 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index cedf4e2..3e5a723 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -152,7 +152,7 @@ static inline int test_time_stamp(u64 delta)
struct ring_buffer_per_cpu {
int cpu;
struct ring_buffer *buffer;
- spinlock_t lock;
+ raw_spinlock_t lock;
struct lock_class_key lock_key;
struct list_head pages;
struct buffer_page *head_page; /* read from head */
@@ -289,7 +289,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)

cpu_buffer->cpu = cpu;
cpu_buffer->buffer = buffer;
- spin_lock_init(&cpu_buffer->lock);
+ cpu_buffer->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
INIT_LIST_HEAD(&cpu_buffer->pages);

page = kzalloc_node(ALIGN(sizeof(*page), cache_line_size()),
@@ -851,8 +851,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
/* See if we shot pass the end of this buffer page */
if (write > BUF_PAGE_SIZE) {
struct buffer_page *next_page = tail_page;
-
- spin_lock_irqsave(&cpu_buffer->lock, flags);
+ raw_local_irq_save(flags);
+ __raw_spin_lock(&cpu_buffer->lock);

rb_inc_page(cpu_buffer, &next_page);

@@ -927,8 +927,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
tail == rb_commit_index(cpu_buffer)) {
rb_set_commit_to_write(cpu_buffer);
}
-
- spin_unlock_irqrestore(&cpu_buffer->lock, flags);
+ __raw_spin_unlock(&cpu_buffer->lock);
+ raw_local_irq_restore(flags);

/* fail and let the caller try again */
return ERR_PTR(-EAGAIN);
@@ -951,7 +951,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
return event;

out_unlock:
- spin_unlock_irqrestore(&cpu_buffer->lock, flags);
+ __raw_spin_unlock(&cpu_buffer->lock);
+ raw_local_irq_restore(flags);
return NULL;
}

@@ -1533,7 +1534,8 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
struct buffer_page *reader = NULL;
unsigned long flags;

- spin_lock_irqsave(&cpu_buffer->lock, flags);
+ raw_local_irq_save(flags);
+ __raw_spin_lock(&cpu_buffer->lock);

again:
reader = cpu_buffer->reader_page;
@@ -1583,7 +1585,8 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
goto again;

out:
- spin_unlock_irqrestore(&cpu_buffer->lock, flags);
+ __raw_spin_unlock(&cpu_buffer->lock);
+ raw_local_irq_restore(flags);

return reader;
}
@@ -1824,9 +1827,11 @@ ring_buffer_read_start(struct ring_buffer *buffer, int cpu)
atomic_inc(&cpu_buffer->record_disabled);
synchronize_sched();

- spin_lock_irqsave(&cpu_buffer->lock, flags);
+ raw_local_irq_save(flags);
+ __raw_spin_lock(&cpu_buffer->lock);
ring_buffer_iter_reset(iter);
- spin_unlock_irqrestore(&cpu_buffer->lock, flags);
+ __raw_spin_unlock(&cpu_buffer->lock);
+ raw_local_irq_restore(flags);

return iter;
}
@@ -1912,11 +1917,13 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
if (!cpu_isset(cpu, buffer->cpumask))
return;

- spin_lock_irqsave(&cpu_buffer->lock, flags);
+ raw_local_irq_save(flags);
+ __raw_spin_lock(&cpu_buffer->lock);

rb_reset_cpu(cpu_buffer);

- spin_unlock_irqrestore(&cpu_buffer->lock, flags);
+ __raw_spin_unlock(&cpu_buffer->lock);
+ raw_local_irq_restore(flags);
}

/**

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