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Subject[PATCH RT 3.18] ring-buffer: Mark irq_work as HARD_IRQ to prevent deadlocks
ftrace may trigger rb_wakeups while holding pi_lock which will also be
requested via trace_...->...->ring_buffer_unlock_commit->...->
irq_work_queue->raise_softirq->try_to_wake_up. This quickly causes
deadlocks when trying to use ftrace under -rt.

Resolve this by marking the ring buffer's irq_work as HARD_IRQ.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

I'm not yet sure if this doesn't push work into hard-irq context that
is better not done there on -rt.

I'm also not sure if there aren't more such cases, given that -rt turns
the default irq_work wakeup policy around. But maybe we are lucky.

kernel/trace/ring_buffer.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f4fbbfc..6a1939e 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1252,6 +1252,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int nr_pages, int cpu)
init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
init_waitqueue_head(&cpu_buffer->irq_work.waiters);
init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
+ cpu_buffer->irq_work.work.flags = IRQ_WORK_HARD_IRQ;

bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
GFP_KERNEL, cpu_to_node(cpu));

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