ring-buffer: Make wake once of ring_buffer_wait() more robust

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().

Link: https://lore.kernel.org/linux-trace-kernel/20240315063115.6cb5d205@gandalf.local.home
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: 7af9ded0c2ca ("ring-buffer: Use wait_event_interruptible() in ring_buffer_wait()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 788d321036bdd16f9f9493c7d0f24ac48dcd49fe..25476ead681b8411f41d713a77603cdf0653b4ad 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -384,6 +384,7 @@ 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;
@@ -753,6 +754,9 @@ 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 */
@@ -881,20 +885,21 @@ 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;
 }
 
 /**
@@ -915,14 +920,9 @@ 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
@@ -944,6 +944,14 @@ 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));