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Subject[tip: locking/core] connector/cn_proc: Protect send_msg() with a local lock
The following commit has been merged into the locking/core branch of tip:

Commit-ID: 3e92fd7bd2b8418b53cb7304855b8b69bedbe2b4
Gitweb: https://git.kernel.org/tip/3e92fd7bd2b8418b53cb7304855b8b69bedbe2b4
Author: Mike Galbraith <umgwanakikbuti@gmail.com>
AuthorDate: Wed, 27 May 2020 22:11:17 +02:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 28 May 2020 10:31:10 +02:00

connector/cn_proc: Protect send_msg() with a local lock

send_msg() disables preemption to avoid out-of-order messages. As the
code inside the preempt disabled section acquires regular spinlocks,
which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
eventually calls into a memory allocator, this conflicts with the RT
semantics.

Convert it to a local_lock which allows RT kernels to substitute them with
a real per CPU lock. On non RT kernels this maps to preempt_disable() as
before. No functional change.

[bigeasy: Patch description]

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200527201119.1692513-6-bigeasy@linutronix.de
---
drivers/connector/cn_proc.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
index d58ce66..646ad38 100644
--- a/drivers/connector/cn_proc.c
+++ b/drivers/connector/cn_proc.c
@@ -18,6 +18,7 @@
#include <linux/pid_namespace.h>

#include <linux/cn_proc.h>
+#include <linux/local_lock.h>

/*
* Size of a cn_msg followed by a proc_event structure. Since the
@@ -38,25 +39,31 @@ static inline struct cn_msg *buffer_to_cn_msg(__u8 *buffer)
static atomic_t proc_event_num_listeners = ATOMIC_INIT(0);
static struct cb_id cn_proc_event_id = { CN_IDX_PROC, CN_VAL_PROC };

-/* proc_event_counts is used as the sequence number of the netlink message */
-static DEFINE_PER_CPU(__u32, proc_event_counts) = { 0 };
+/* local_event.count is used as the sequence number of the netlink message */
+struct local_event {
+ local_lock_t lock;
+ __u32 count;
+};
+static DEFINE_PER_CPU(struct local_event, local_event) = {
+ .lock = INIT_LOCAL_LOCK(lock),
+};

static inline void send_msg(struct cn_msg *msg)
{
- preempt_disable();
+ local_lock(&local_event.lock);

- msg->seq = __this_cpu_inc_return(proc_event_counts) - 1;
+ msg->seq = __this_cpu_inc_return(local_event.count) - 1;
((struct proc_event *)msg->data)->cpu = smp_processor_id();

/*
- * Preemption remains disabled during send to ensure the messages are
- * ordered according to their sequence numbers.
+ * local_lock() disables preemption during send to ensure the messages
+ * are ordered according to their sequence numbers.
*
* If cn_netlink_send() fails, the data is not sent.
*/
cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_NOWAIT);

- preempt_enable();
+ local_unlock(&local_event.lock);
}

void proc_fork_connector(struct task_struct *task)
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