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sched: Prevent recursion in io_schedule()

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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>
This commit is contained in:
NeilBrown 2015-02-13 15:49:17 +11:00 committed by Ingo Molnar
parent bc9560155f
commit 9cff8adeaa
2 changed files with 20 additions and 23 deletions
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10
include/linux/sched.h
View File

@ -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;

33
kernel/sched/core.c
View File

@ -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;
delayacct_blkio_start();
atomic_inc(&rq->nr_iowait);
blk_flush_plug(current);
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);
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.