[Devel] [PATCH RHEL7 COMMIT] ms/tracing: Fix race in trace_open and buffer resize call

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.6.1.vz7.171.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.6.1.vz7.171.2
------>
commit d0b2f7cda662987572c5f84e7a8831d8c1135bd1
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Thu Dec 3 15:31:57 2020 +0300

    ms/tracing: Fix race in trace_open and buffer resize call
    
    Author: Gaurav Kohli <gkohli at codeaurora.org>
    backported upstream commit bbeb97464eefc65f506084fd9f18f21653e01137
    
        Below race can come, if trace_open and resize of
        cpu buffer is running parallely on different cpus
        CPUX                                CPUY
                                        ring_buffer_resize
                                        atomic_read(&buffer->resize_disabled)
        tracing_open
        tracing_reset_online_cpus
        ring_buffer_reset_cpu
        rb_reset_cpu
                                        rb_update_pages
                                        remove/insert pages
        resetting pointer
    
        This race can cause data abort or some times infinte loop in
        rb_remove_pages and rb_insert_pages while checking pages
        for sanity.
    
        Take buffer lock to fix this.
    
        Signed-off-by: Gaurav Kohli <gkohli at codeaurora.org>
        Signed-off-by: Steven Rostedt (VMware) <rostedt at goodmis.org>
    
    backport changes: ring_buffer_reset_cpu() fixed only
    ring_buffer_reset_online_cpus() is not present in this kernel
    https://jira.sw.ru/browse/PSBM-122343
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 kernel/trace/ring_buffer.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8b3df28..7b1afd1 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4170,6 +4170,9 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return;
 
+	/* prevent another thread from changing buffer sizes */
+	mutex_lock(&buffer->mutex);
+
 	atomic_inc(&buffer->resize_disabled);
 	atomic_inc(&cpu_buffer->record_disabled);
 
@@ -4192,6 +4195,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
 
 	atomic_dec(&cpu_buffer->record_disabled);
 	atomic_dec(&buffer->resize_disabled);
+
+	mutex_unlock(&buffer->mutex);
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);