tracing: kdb: Fix ftdump to not sleep

[ Upstream commit 31b265b3baaf55f209229888b7ffea523ddab366 ]

As reported back in 2016-11 [1], the "ftdump" kdb command triggers a
BUG for "sleeping function called from invalid context".

kdb's "ftdump" command wants to call ring_buffer_read_prepare() in
atomic context.  A very simple solution for this is to add allocation
flags to ring_buffer_read_prepare() so kdb can call it without
triggering the allocation error.  This patch does that.

Note that in the original email thread about this, it was suggested
that perhaps the solution for kdb was to either preallocate the buffer
ahead of time or create our own iterator.  I'm hoping that this
alternative of adding allocation flags to ring_buffer_read_prepare()
can be considered since it means I don't need to duplicate more of the
core trace code into "trace_kdb.c" (for either creating my own
iterator or re-preparing a ring allocator whose memory was already
allocated).

NOTE: another option for kdb is to actually figure out how to make it
reuse the existing ftrace_dump() function and totally eliminate the
duplication.  This sounds very appealing and actually works (the "sr
z" command can be seen to properly dump the ftrace buffer).  The
downside here is that ftrace_dump() fully consumes the trace buffer.
Unless that is changed I'd rather not use it because it means "ftdump
| grep xyz" won't be very useful to search the ftrace buffer since it
will throw away the whole trace on the first grep.  A future patch to
dump only the last few lines of the buffer will also be hard to
implement.

[1] https://lkml.kernel.org/r/20161117191605.GA21459@google.com

Link: http://lkml.kernel.org/r/20190308193205.213659-1-dianders@chromium.org
Reported-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 0940fda59872c039d8a555bc533113648a0b1d08..941bfd9b3c89cd1dfd1931fdf9629de21771d523 100644 (file)
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -128,7 +128,7 @@ ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts,
                    unsigned long *lost_events);
 
 struct ring_buffer_iter *
-ring_buffer_read_prepare(struct ring_buffer *buffer, int cpu);
+ring_buffer_read_prepare(struct ring_buffer *buffer, int cpu, gfp_t flags);
 void ring_buffer_read_prepare_sync(void);
 void ring_buffer_read_start(struct ring_buffer_iter *iter);
 void ring_buffer_read_finish(struct ring_buffer_iter *iter);

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 65bd4616220db72b5e817cd0d1fdf1d4958a9501..34b4c32b069200e5ceb8164a1a6a7aef96c9499d 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4141,6 +4141,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_consume);
  * ring_buffer_read_prepare - Prepare for a non consuming read of the buffer
  * @buffer: The ring buffer to read from
  * @cpu: The cpu buffer to iterate over
+ * @flags: gfp flags to use for memory allocation
  *
  * This performs the initial preparations necessary to iterate
  * through the buffer.  Memory is allocated, buffer recording
@@ -4158,7 +4159,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_consume);
  * This overall must be paired with ring_buffer_read_finish.
  */
 struct ring_buffer_iter *
-ring_buffer_read_prepare(struct ring_buffer *buffer, int cpu)
+ring_buffer_read_prepare(struct ring_buffer *buffer, int cpu, gfp_t flags)
 {
        struct ring_buffer_per_cpu *cpu_buffer;
        struct ring_buffer_iter *iter;
@@ -4166,7 +4167,7 @@ ring_buffer_read_prepare(struct ring_buffer *buffer, int cpu)
        if (!cpumask_test_cpu(cpu, buffer->cpumask))
                return NULL;
 
-       iter = kmalloc(sizeof(*iter), GFP_KERNEL);
+       iter = kmalloc(sizeof(*iter), flags);
        if (!iter)
                return NULL;
 

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 1f96b292df3190eaa8d295ef4d650415935b5ab5..c65cea71d1ee5e1d44fb8979bc1798000632650e 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3903,7 +3903,8 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
        if (iter->cpu_file == RING_BUFFER_ALL_CPUS) {
                for_each_tracing_cpu(cpu) {
                        iter->buffer_iter[cpu] =
-                               ring_buffer_read_prepare(iter->trace_buffer->buffer, cpu);
+                               ring_buffer_read_prepare(iter->trace_buffer->buffer,
+                                                        cpu, GFP_KERNEL);
                }
                ring_buffer_read_prepare_sync();
                for_each_tracing_cpu(cpu) {
@@ -3913,7 +3914,8 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
        } else {
                cpu = iter->cpu_file;
                iter->buffer_iter[cpu] =
-                       ring_buffer_read_prepare(iter->trace_buffer->buffer, cpu);
+                       ring_buffer_read_prepare(iter->trace_buffer->buffer,
+                                                cpu, GFP_KERNEL);
                ring_buffer_read_prepare_sync();
                ring_buffer_read_start(iter->buffer_iter[cpu]);
                tracing_iter_reset(iter, cpu);

diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index d953c163a0794f5cd4578ae6957f0645ff1eaa0d..810d78a8d14c76b02efa3dff91b63e52e4c66feb 100644 (file)
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -51,14 +51,16 @@ static void ftrace_dump_buf(int skip_lines, long cpu_file)
        if (cpu_file == RING_BUFFER_ALL_CPUS) {
                for_each_tracing_cpu(cpu) {
                        iter.buffer_iter[cpu] =
-                       ring_buffer_read_prepare(iter.trace_buffer->buffer, cpu);
+                       ring_buffer_read_prepare(iter.trace_buffer->buffer,
+                                                cpu, GFP_ATOMIC);
                        ring_buffer_read_start(iter.buffer_iter[cpu]);
                        tracing_iter_reset(&iter, cpu);
                }
        } else {
                iter.cpu_file = cpu_file;
                iter.buffer_iter[cpu_file] =
-                       ring_buffer_read_prepare(iter.trace_buffer->buffer, cpu_file);
+                       ring_buffer_read_prepare(iter.trace_buffer->buffer,
+                                                cpu_file, GFP_ATOMIC);
                ring_buffer_read_start(iter.buffer_iter[cpu_file]);
                tracing_iter_reset(&iter, cpu_file);
        }