[PATCH 3.15 082/109] ring-buffer: Fix polling on trace_pipe
From: Greg Kroah-Hartman
Date: Sat Jul 26 2014 - 15:23:46 EST
3.15-stable review patch. If anyone has any objections, please let me know.
From: Martin Lau <kafai@xxxxxx>
commit 97b8ee845393701edc06e27ccec2876ff9596019 upstream.
ring_buffer_poll_wait() should always put the poll_table to its wait_queue
even there is immediate data available. Otherwise, the following epoll and
read sequence will eventually hang forever:
1. Put some data to make the trace_pipe ring_buffer read ready first
2. epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, ee)
4. read(trace_pipe_fd) till EAGAIN
5. Add some more data to the trace_pipe ring_buffer
6. epoll_wait() -> this epoll_wait() will block forever
~ During the epoll_ctl(efd, EPOLL_CTL_ADD,...) call in step 2,
ring_buffer_poll_wait() returns immediately without adding poll_table,
which has poll_table->_qproc pointing to ep_poll_callback(), to its
~ During the epoll_wait() call in step 3 and step 6,
ring_buffer_poll_wait() cannot add ep_poll_callback() to its wait_queue
because the poll_table->_qproc is NULL and it is how epoll works.
~ When there is new data available in step 6, ring_buffer does not know
it has to call ep_poll_callback() because it is not in its wait queue.
Hence, block forever.
Other poll implementation seems to call poll_wait() unconditionally as the very
first thing to do. For example, tcp_poll() in tcp.c.
Fixes: 2a2cc8f7c4d0 "ftrace: allow the event pipe to be polled"
Reviewed-by: Chris Mason <clm@xxxxxx>
Signed-off-by: Martin Lau <kafai@xxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
kernel/trace/ring_buffer.c | 4 ----
1 file changed, 4 deletions(-)
@@ -616,10 +616,6 @@ int ring_buffer_poll_wait(struct ring_bu
struct ring_buffer_per_cpu *cpu_buffer;
struct rb_irq_work *work;
- if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) ||
- (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))
- return POLLIN | POLLRDNORM;
if (cpu == RING_BUFFER_ALL_CPUS)
work = &buffer->irq_work;
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