[PATCH 5.4 062/215] ring-buffer: Fix resetting of shortest_full

Greg Kroah-Hartman
2024-04-11T10:17:09Z

From: Steven Rostedt (Google) <rostedt@goodmis.org>

[ Upstream commit 68282dd930ea38b068ce2c109d12405f40df3f93 ]

The "shortest_full" variable is used to keep track of the waiter that is
waiting for the smallest amount on the ring buffer before being woken up.
When a tasks waits on the ring buffer, it passes in a "full" value that is
a percentage. 0 means wake up on any data. 1-100 means wake up from 1% to
100% full buffer.

As all waiters are on the same wait queue, the wake up happens for the
waiter with the smallest percentage.

The problem is that the smallest_full on the cpu_buffer that stores the
smallest amount doesn't get reset when all the waiters are woken up. It
does get reset when the ring buffer is reset (echo > /sys/kernel/tracing/trace).

This means that tasks may be woken up more often then when they want to
be. Instead, have the shortest_full field get reset just before waking up
all the tasks. If the tasks wait again, they will update the shortest_full
before sleeping.

Also add locking around setting of shortest_full in the poll logic, and
change "work" to "rbwork" to match the variable name for rb_irq_work
structures that are used in other places.

Link: https://lore.kernel.org/linux-trace-kernel/20240308202431.948914369@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linke li <lilinke99@qq.com>
Cc: Rabin Vincent <rabin@rab.in>
Fixes: 2c2b0a78b3739 ("ring-buffer: Add percentage of ring buffer full to wake up reader")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Stable-dep-of: 8145f1c35fa6 ("ring-buffer: Fix full_waiters_pending in poll")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/trace/ring_buffer.c | 30 +++++++++++++++++++++++-------
1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index ed505c6de7cae..a9c90088af780 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -601,8 +601,19 @@ static void rb_wake_up_waiters(struct irq_work *work)

wake_up_all(&rbwork->waiters);
if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
+ /* Only cpu_buffer sets the above flags */
+ struct ring_buffer_per_cpu *cpu_buffer =
+ container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
+
+ /* Called from interrupt context */
+ raw_spin_lock(&cpu_buffer->reader_lock);
rbwork->wakeup_full = false;
rbwork->full_waiters_pending = false;
+
+ /* Waking up all waiters, they will reset the shortest full */
+ cpu_buffer->shortest_full = 0;
+ raw_spin_unlock(&cpu_buffer->reader_lock);
+
wake_up_all(&rbwork->full_waiters);
}
}
@@ -731,28 +742,33 @@ __poll_t ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
struct file *filp, poll_table *poll_table, int full)
{
struct ring_buffer_per_cpu *cpu_buffer;
- struct rb_irq_work *work;
+ struct rb_irq_work *rbwork;

if (cpu == RING_BUFFER_ALL_CPUS) {
- work = &buffer->irq_work;
+ rbwork = &buffer->irq_work;
full = 0;
} else {
if (!cpumask_test_cpu(cpu, buffer->cpumask))
return EPOLLERR;

cpu_buffer = buffer->buffers[cpu];
- work = &cpu_buffer->irq_work;
+ rbwork = &cpu_buffer->irq_work;
}

if (full) {
- poll_wait(filp, &work->full_waiters, poll_table);
- work->full_waiters_pending = true;
+ unsigned long flags;
+
+ poll_wait(filp, &rbwork->full_waiters, poll_table);
+
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ rbwork->full_waiters_pending = true;
if (!cpu_buffer->shortest_full ||
cpu_buffer->shortest_full > full)
cpu_buffer->shortest_full = full;
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
} else {
- poll_wait(filp, &work->waiters, poll_table);
- work->waiters_pending = true;
+ poll_wait(filp, &rbwork->waiters, poll_table);
+ rbwork->waiters_pending = true;
}

/*
--
2.43.0