Commit 3e92fd7b authored by Mike Galbraith's avatar Mike Galbraith Committed by Ingo Molnar
Browse files

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: default avatarMike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200527201119.1692513-6-bigeasy@linutronix.de
parent fd56200a

drivers/connector/cn_proc.c

+14 −7
Original line number Diff line number Diff line
@@ -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)