Re: [tracing] cd8f62b481: BUG:sleeping_function_called_from_invalid_context_at_mm/slab.h

[Steven Rostedt]

On Wed, 1 Apr 2020 10:21:12 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 6519b7afc499..7f1466253ca8 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3487,6 +3487,14 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
>  	 */
>  	if (iter->ent && iter->ent != iter->temp) {
>  		if (!iter->temp || iter->temp_size < iter->ent_size) {
> +			/*
> +			 * This function is only used to add markers between
> +			 * events that are far apart (see trace_print_lat_context()),
> +			 * but if this is called in an atomic context (like NMIs)
> +			 * we can't call kmalloc(), thus just return NULL.
> +			 */
> +			if (in_atomic() || irqs_disabled())
> +				return NULL;
>  			kfree(iter->temp);
>  			iter->temp = kmalloc(iter->ent_size, GFP_KERNEL);
>  			if (!iter->temp)

Peter informed me on IRC not to use in_atomic() as it doesn't catch
spin_locks when CONFIG_PREEMPT is not defined.

As the issue is just with ftrace_dump(), I'll have it use a static buffer
instead of 128 bytes. Which should be big enough for most events, and if
not, then it will just miss the markers.

-- Steve

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6519b7afc499..8c9d6a75abbf 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3472,6 +3472,8 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
 	return next;
 }
 
+#define IGNORE_TEMP		((struct trace_iterator *)-1L)
+
 /* Find the next real entry, without updating the iterator itself */
 struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
 					  int *ent_cpu, u64 *ent_ts)
@@ -3480,6 +3482,17 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
 	int ent_size = iter->ent_size;
 	struct trace_entry *entry;
 
+	/*
+	 * This function is only used to add markers between
+	 * events that are far apart (see trace_print_lat_context()),
+	 * but if this is called in an atomic context (like NMIs)
+	 * kmalloc() can't be called.
+	 * That happens via ftrace_dump() which will initialize
+	 * iter->temp to IGNORE_TEMP. In such a case, just return NULL.
+	 */
+	if (iter->temp == IGNORE_TEMP)
+		return NULL;
+
 	/*
 	 * The __find_next_entry() may call peek_next_entry(), which may
 	 * call ring_buffer_peek() that may make the contents of iter->ent
@@ -9203,6 +9216,8 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
 
 	/* Simulate the iterator */
 	trace_init_global_iter(&iter);
+	/* Force not using the temp buffer */
+	iter.temp = IGNORE_TEMP;
 
 	for_each_tracing_cpu(cpu) {
 		atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);