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tracing: Do not busy wait in buffer splice

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On a !PREEMPT kernel, attempting to use trace-cmd results in a soft
lockup:

 # trace-cmd record -e raw_syscalls:* -F false
 NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [trace-cmd:61]
 ...
 Call Trace:
  [<ffffffff8105b580>] ? __wake_up_common+0x90/0x90
  [<ffffffff81092e25>] wait_on_pipe+0x35/0x40
  [<ffffffff810936e3>] tracing_buffers_splice_read+0x2e3/0x3c0
  [<ffffffff81093300>] ? tracing_stats_read+0x2a0/0x2a0
  [<ffffffff812d10ab>] ? _raw_spin_unlock+0x2b/0x40
  [<ffffffff810dc87b>] ? do_read_fault+0x21b/0x290
  [<ffffffff810de56a>] ? handle_mm_fault+0x2ba/0xbd0
  [<ffffffff81095c80>] ? trace_event_buffer_lock_reserve+0x40/0x80
  [<ffffffff810951e2>] ? trace_buffer_lock_reserve+0x22/0x60
  [<ffffffff81095c80>] ? trace_event_buffer_lock_reserve+0x40/0x80
  [<ffffffff8112415d>] do_splice_to+0x6d/0x90
  [<ffffffff81126971>] SyS_splice+0x7c1/0x800
  [<ffffffff812d1edd>] tracesys_phase2+0xd3/0xd8

The problem is this: tracing_buffers_splice_read() calls
ring_buffer_wait() to wait for data in the ring buffers.  The buffers
are not empty so ring_buffer_wait() returns immediately.  But
tracing_buffers_splice_read() calls ring_buffer_read_page() with full=1,
meaning it only wants to read a full page.  When the full page is not
available, tracing_buffers_splice_read() tries to wait again with
ring_buffer_wait(), which again returns immediately, and so on.

Fix this by adding a "full" argument to ring_buffer_wait() which will
make ring_buffer_wait() wait until the writer has left the reader's
page, i.e.  until full-page reads will succeed.

Link: http://lkml.kernel.org/r/1415645194-25379-1-git-send-email-rabin@rab.in

Cc: stable@vger.kernel.org # 3.16+
Fixes: b1169cc69b ("tracing: Remove mock up poll wait function")
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Rabin Vincent 2014-11-10 19:46:34 +01:00 • committed by Steven Rostedt
commit e30f53aad2
3 changed files with 62 additions and 44 deletions
Download patch file Download diff file Expand all files Collapse all files

2
include/linux/ring_buffer.h
View file

@ -97,7 +97,7 @@ __ring_buffer_alloc(unsigned long size, unsigned flags, struct lock_class_key *k
__ring_buffer_alloc((size), (flags), &__key); \
})
int ring_buffer_wait(struct ring_buffer *buffer, int cpu);
int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full);
int ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
struct file *filp, poll_table *poll_table);

81
kernel/trace/ring_buffer.c
View file

@ -538,16 +538,18 @@ static void rb_wake_up_waiters(struct irq_work *work)
* ring_buffer_wait - wait for input to the ring buffer
* @buffer: buffer to wait on
* @cpu: the cpu buffer to wait on
* @full: wait until a full page is available, if @cpu != RING_BUFFER_ALL_CPUS
*
* If @cpu == RING_BUFFER_ALL_CPUS then the task will wake up as soon
* as data is added to any of the @buffer's cpu buffers. Otherwise
* it will wait for data to be added to a specific cpu buffer.
*/
int ring_buffer_wait(struct ring_buffer *buffer, int cpu)
int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full)
{
struct ring_buffer_per_cpu *cpu_buffer;
struct ring_buffer_per_cpu *uninitialized_var(cpu_buffer);
DEFINE_WAIT(wait);
struct rb_irq_work *work;
int ret = 0;
/*
* Depending on what the caller is waiting for, either any
@ -564,36 +566,61 @@ int ring_buffer_wait(struct ring_buffer *buffer, int cpu)
}
prepare_to_wait(&work->waiters, &wait, TASK_INTERRUPTIBLE);
while (true) {
prepare_to_wait(&work->waiters, &wait, TASK_INTERRUPTIBLE);
/*
* The events can happen in critical sections where
* checking a work queue can cause deadlocks.
* After adding a task to the queue, this flag is set
* only to notify events to try to wake up the queue
* using irq_work.
*
* We don't clear it even if the buffer is no longer
* empty. The flag only causes the next event to run
* irq_work to do the work queue wake up. The worse
* that can happen if we race with !trace_empty() is that
* an event will cause an irq_work to try to wake up
* an empty queue.
*
* There's no reason to protect this flag either, as
* the work queue and irq_work logic will do the necessary
* synchronization for the wake ups. The only thing
* that is necessary is that the wake up happens after
* a task has been queued. It's OK for spurious wake ups.
*/
work->waiters_pending = true;
/*
* The events can happen in critical sections where
* checking a work queue can cause deadlocks.
* After adding a task to the queue, this flag is set
* only to notify events to try to wake up the queue
* using irq_work.
*
* We don't clear it even if the buffer is no longer
* empty. The flag only causes the next event to run
* irq_work to do the work queue wake up. The worse
* that can happen if we race with !trace_empty() is that
* an event will cause an irq_work to try to wake up
* an empty queue.
*
* There's no reason to protect this flag either, as
* the work queue and irq_work logic will do the necessary
* synchronization for the wake ups. The only thing
* that is necessary is that the wake up happens after
* a task has been queued. It's OK for spurious wake ups.
*/
work->waiters_pending = true;
if (signal_pending(current)) {
ret = -EINTR;
break;
}
if (cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer))
break;
if (cpu != RING_BUFFER_ALL_CPUS &&
!ring_buffer_empty_cpu(buffer, cpu)) {
unsigned long flags;
bool pagebusy;
if (!full)
break;
raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
if (!pagebusy)
break;
}
if ((cpu == RING_BUFFER_ALL_CPUS && ring_buffer_empty(buffer)) ||
(cpu != RING_BUFFER_ALL_CPUS && ring_buffer_empty_cpu(buffer, cpu)))
schedule();
}
finish_wait(&work->waiters, &wait);
return 0;
return ret;
}
/**

23
kernel/trace/trace.c
View file

@ -1076,13 +1076,14 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
}
#endif /* CONFIG_TRACER_MAX_TRACE */
static int wait_on_pipe(struct trace_iterator *iter)
static int wait_on_pipe(struct trace_iterator *iter, bool full)
{
/* Iterators are static, they should be filled or empty */
if (trace_buffer_iter(iter, iter->cpu_file))
return 0;
return ring_buffer_wait(iter->trace_buffer->buffer, iter->cpu_file);
return ring_buffer_wait(iter->trace_buffer->buffer, iter->cpu_file,
full);
}
#ifdef CONFIG_FTRACE_STARTUP_TEST
@ -4434,15 +4435,12 @@ static int tracing_wait_pipe(struct file *filp)
mutex_unlock(&iter->mutex);
ret = wait_on_pipe(iter);
ret = wait_on_pipe(iter, false);
mutex_lock(&iter->mutex);
if (ret)
return ret;
if (signal_pending(current))
return -EINTR;
}
return 1;
@ -5372,16 +5370,12 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
goto out_unlock;
}
mutex_unlock(&trace_types_lock);
ret = wait_on_pipe(iter);
ret = wait_on_pipe(iter, false);
mutex_lock(&trace_types_lock);
if (ret) {
size = ret;
goto out_unlock;
}
if (signal_pending(current)) {
size = -EINTR;
goto out_unlock;
}
goto again;
}
size = 0;
@ -5587,14 +5581,11 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
goto out;
}
mutex_unlock(&trace_types_lock);
ret = wait_on_pipe(iter);
ret = wait_on_pipe(iter, true);
mutex_lock(&trace_types_lock);
if (ret)
goto out;
if (signal_pending(current)) {
ret = -EINTR;
goto out;
}
goto again;
}