[PATCH] Fix current_io_context() vs set_task_ioprio() race

I know nothing about io scheduler, but I suspect set_task_ioprio() is not safe.

current_io_context() initializes "struct io_context", then sets ->io_context.
set_task_ioprio() running on another cpu may see the changes out of order, so
->set_ioprio(ioc) may use io_context which was not initialized properly.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/ll_rw_blk.c

@@ -3628,6 +3628,8 @@ struct io_context *current_io_context(gfp_t gfp_flags)
 		ret->nr_batch_requests = 0; /* because this is 0 */
 		ret->aic = NULL;
 		ret->cic_root.rb_node = NULL;
+		/* make sure set_task_ioprio() sees the settings above */
+		smp_wmb();
 		tsk->io_context = ret;
 	}
 

fs/ioprio.c

@@ -44,6 +44,9 @@ static int set_task_ioprio(struct task_struct *task, int ioprio)
 	task->ioprio = ioprio;
 
 	ioc = task->io_context;
+	/* see wmb() in current_io_context() */
+	smp_read_barrier_depends();
+
 	if (ioc && ioc->set_ioprio)
 		ioc->set_ioprio(ioc, ioprio);