From: "Steven Rostedt (Google)" <rost...@goodmis.org>
The default behavior of ring_buffer_wait() when passed a NULL "cond"
parameter is to exit the function the first time it is woken up. The
current implementation uses a counter that starts at zero and when it is
greater than one it exits the wait_event_interruptible().
But this relies on the internal working of wait_event_interruptible() as
that code basically has:
if (cond)
return;
prepare_to_wait();
if (!cond)
schedule();
finish_wait();
That is, cond is called twice before it sleeps. The default cond of
ring_buffer_wait() needs to account for that and wait for its counter to
increment twice before exiting.
Instead, use the seq/atomic_inc logic that is used by the tracing code
that calls this function. Add an atomic_t seq to rb_irq_work and when cond
is NULL, have the default callback take a descriptor as its data that
holds the rbwork and the value of the seq when it started.
The wakeups will now increment the rbwork->seq and the cond callback will
simply check if that number is different, and no longer have to rely on
the implementation of wait_event_interruptible().
Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
---
kernel/trace/ring_buffer.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f0f3577339b3..c8ff71d16a6e 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -388,6 +388,7 @@ struct rb_irq_work {
struct irq_work work;
wait_queue_head_t waiters;
wait_queue_head_t full_waiters;
+ atomic_t seq;
bool waiters_pending;
bool full_waiters_pending;
bool wakeup_full;
@@ -763,6 +764,9 @@ static void rb_wake_up_waiters(struct irq_work *work)
{
struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work,
work);
+ /* For waiters waiting for the first wake up */
+ (void)atomic_fetch_inc_release(&rbwork->seq);
+
wake_up_all(&rbwork->waiters);
if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
/* Only cpu_buffer sets the above flags */
@@ -891,20 +895,21 @@ rb_wait_cond(struct rb_irq_work *rbwork, struct
trace_buffer *buffer,
return false;
}
+struct rb_wait_data {
+ struct rb_irq_work *irq_work;
+ int seq;
+};
+
/*
* The default wait condition for ring_buffer_wait() is to just to exit the
* wait loop the first time it is woken up.
*/
static bool rb_wait_once(void *data)
{
- long *once = data;
+ struct rb_wait_data *rdata = data;
+ struct rb_irq_work *rbwork = rdata->irq_work;
- /* wait_event() actually calls this twice before scheduling*/
- if (*once > 1)
- return true;
-
- (*once)++;
- return false;
+ return atomic_read_acquire(&rbwork->seq) != rdata->seq;
}
/**
@@ -925,14 +930,9 @@ int ring_buffer_wait(struct trace_buffer *buffer, int cpu,
int full,
struct ring_buffer_per_cpu *cpu_buffer;
struct wait_queue_head *waitq;
struct rb_irq_work *rbwork;
- long once = 0;
+ struct rb_wait_data rdata;
int ret = 0;
- if (!cond) {
- cond = rb_wait_once;
- data = &once;
- }
-
/*
* Depending on what the caller is waiting for, either any
* data in any cpu buffer, or a specific buffer, put the
@@ -954,6 +954,14 @@ int ring_buffer_wait(struct trace_buffer *buffer, int cpu,
int full,
else
waitq = &rbwork->waiters;
+ /* Set up to exit loop as soon as it is woken */
+ if (!cond) {
+ cond = rb_wait_once;
+ rdata.irq_work = rbwork;
+ rdata.seq = atomic_read_acquire(&rbwork->seq);
+ data = &rdata;
+ }
+
ret = wait_event_interruptible((*waitq),
rb_wait_cond(rbwork, buffer, cpu, full, cond,
data));
--
2.43.0
