sched: Prevent recursion in io_schedule()

io_schedule() calls blk_flush_plug() which, depending on the
contents of current->plug, can initiate arbitrary blk-io requests.

Note that this contrasts with blk_schedule_flush_plug() which requires
all non-trivial work to be handed off to a separate thread.

This makes it possible for io_schedule() to recurse, and initiating
block requests could possibly call mempool_alloc() which, in times of
memory pressure, uses io_schedule().

Apart from any stack usage issues, io_schedule() will not behave
correctly when called recursively as delayacct_blkio_start() does
not allow for repeated calls.

So:
 - use ->in_iowait to detect recursion.  Set it earlier, and restore
   it to the old value.
 - move the call to "raw_rq" after the call to blk_flush_plug().
   As this is some sort of per-cpu thing, we want some chance that
   we are on the right CPU
 - When io_schedule() is called recurively, use blk_schedule_flush_plug()
   which cannot further recurse.
 - as this makes io_schedule() a lot more complex and as io_schedule()
   must match io_schedule_timeout(), but all the changes in io_schedule_timeout()
   and make io_schedule a simple wrapper for that.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ Moved the now rudimentary io_schedule() into sched.h. ]
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tony Battersby <tonyb@cybernetics.com>
Link: http://lkml.kernel.org/r/20150213162600.059fffb2@notabene.brown
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8db31ef..cb5cdc7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -363,9 +363,6 @@
  */
 extern void show_stack(struct task_struct *task, unsigned long *sp);
 
-void io_schedule(void);
-long io_schedule_timeout(long timeout);
-
 extern void cpu_init (void);
 extern void trap_init(void);
 extern void update_process_times(int user);
@@ -422,6 +419,13 @@
 asmlinkage void schedule(void);
 extern void schedule_preempt_disabled(void);
 
+extern long io_schedule_timeout(long timeout);
+
+static inline void io_schedule(void)
+{
+	io_schedule_timeout(MAX_SCHEDULE_TIMEOUT);
+}
+
 struct nsproxy;
 struct user_namespace;