Fix: Don't nest get online cpus

Since the cpu hotplug refactoring in the Linux kernel, CPU hotplug
"online cpus" read lock cannot be nested anymore.

Fix this by disabling preemption around the section instead.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/lib/ringbuffer/ring_buffer_frontend.c b/lib/ringbuffer/ring_buffer_frontend.c
index c9ab43f92eee39229f1c3892d1c9610691bec614..bdd31add4382dfb0a4cf41cfc3b9f124075a4df0 100644 (file)
--- a/lib/ringbuffer/ring_buffer_frontend.c
+++ b/lib/ringbuffer/ring_buffer_frontend.c
@@ -1885,16 +1885,14 @@ static void _lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf,
        }
 
        /*
-        * Taking lock on CPU hotplug to ensure two things: first, that the
+        * Disabling preemption ensures two things: first, that the
         * target cpu is not taken concurrently offline while we are within
-        * smp_call_function_single() (I don't trust that get_cpu() on the
-        * _local_ CPU actually inhibit CPU hotplug for the _remote_ CPU (to be
-        * confirmed)). Secondly, if it happens that the CPU is not online, our
-        * own call to lib_ring_buffer_switch_slow() needs to be protected from
-        * CPU hotplug handlers, which can also perform a remote subbuffer
-        * switch.
+        * smp_call_function_single(). Secondly, if it happens that the
+        * CPU is not online, our own call to lib_ring_buffer_switch_slow()
+        * needs to be protected from CPU hotplug handlers, which can
+        * also perform a remote subbuffer switch.
         */
-       get_online_cpus();
+       preempt_disable();
        param.buf = buf;
        param.mode = mode;
        ret = smp_call_function_single(buf->backend.cpu,
@@ -1903,7 +1901,7 @@ static void _lib_ring_buffer_switch_remote(struct lib_ring_buffer *buf,
                /* Remote CPU is offline, do it ourself. */
                lib_ring_buffer_switch_slow(buf, mode);
        }
-       put_online_cpus();
+       preempt_enable();
 }
 
 /* Switch sub-buffer if current sub-buffer is non-empty. */