[tip:sched/core] sched: Prevent recursion in io_schedule()

tip-bot for NeilBrown
2015-02-18T17:07:59Z

Commit-ID: 9cff8adeaa34b5d2802f03f89803da57856b3b72
Gitweb: http://git.kernel.org/tip/9cff8adeaa34b5d2802f03f89803da57856b3b72
Author: NeilBrown <neilb@suse.de>
AuthorDate: Fri, 13 Feb 2015 15:49:17 +1100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 18 Feb 2015 14:27:44 +0100

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 | 10 +++++++---
kernel/sched/core.c | 31 ++++++++++++-------------------
2 files changed, 19 insertions(+), 22 deletions(-)

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_regs(struct pt_regs *);
*/
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 @@ extern signed long schedule_timeout_uninterruptible(signed long timeout);
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;

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c314000..daaea92 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4358,36 +4358,29 @@ EXPORT_SYMBOL_GPL(yield_to);
* This task is about to go to sleep on IO. Increment rq->nr_iowait so
* that process accounting knows that this is a task in IO wait state.
*/
-void __sched io_schedule(void)
-{
- struct rq *rq = raw_rq();
-
- delayacct_blkio_start();
- atomic_inc(&rq->nr_iowait);
- blk_flush_plug(current);
- current->in_iowait = 1;
- schedule();
- current->in_iowait = 0;
- atomic_dec(&rq->nr_iowait);
- delayacct_blkio_end();
-}
-EXPORT_SYMBOL(io_schedule);
-
long __sched io_schedule_timeout(long timeout)
{
- struct rq *rq = raw_rq();
+ int old_iowait = current->in_iowait;
+ struct rq *rq;
long ret;

+ current->in_iowait = 1;
+ if (old_iowait)
+ blk_schedule_flush_plug(current);
+ else
+ blk_flush_plug(current);
+
delayacct_blkio_start();
+ rq = raw_rq();
atomic_inc(&rq->nr_iowait);
- blk_flush_plug(current);
- current->in_iowait = 1;
ret = schedule_timeout(timeout);
- current->in_iowait = 0;
+ current->in_iowait = old_iowait;
atomic_dec(&rq->nr_iowait);
delayacct_blkio_end();
+
return ret;
}
+EXPORT_SYMBOL(io_schedule_timeout);

/**
* sys_sched_get_priority_max - return maximum RT priority.