On Wed, Apr 06 2016, Shaohua Li wrote:
> On Tue, Apr 05, 2016 at 03:36:57PM +0200, Lars Ellenberg wrote:
>> blk_check_plugged() will return a pointer
>> to an object linked on current->plug->cb_list.
>>
>> That list may "at any time" be implicitly cleared by
>> blk_flush_plug_list()
>> flush_plug_callbacks()
>> either as a result of blk_finish_plug(),
>> or implicitly by schedule() [and maybe other implicit mechanisms?]
>>
>> If there is no protection against an implicit unplug
>> between the call to blk_check_plug() and using its return value,
>> that implicit unplug may have already happened,
>> even before the plug is actually initialized or populated,
>> and we may be using a pointer to already free()d data.
>
> This isn't correct. flush plug is never called in preemption, which is designed
> only called when the task is going to sleep. See sched_submit_work. Am I
> missing anything?
Ahh yes, thanks.
Only two places call blk_schedule_flush_plug().
One is io_schedule_timeout() which must be called explicitly.
There other is, as you say, sched_submit_work(). It starts:
static inline void sched_submit_work(struct task_struct *tsk)
{
if (!tsk->state || tsk_is_pi_blocked(tsk))
return;
so if the task is runnable, then as
include/linux/sched.h:#define TASK_RUNNING 0
it will never call blk_schedule_flush_plug().
So I don't think you are missing anything, we were.
Lars: have your concerns been relieved or do you still have reason to
think there is a problem?
Thanks,
NeilBrown
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