linux-kernel - Re: Multi-partition block layer behaviour

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Date:	Thu, 27 Oct 2011 08:42:41 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Tiju Jacob <jacobtiju@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Multi-partition block layer behaviour

On Wed, 2011-10-26 at 18:10 +0800, Tiju Jacob wrote:
> >> 1. When an I/O request is made to the filesystem, process 'A' acquires
> >> a mutex FS lock and a mutex block driver lock.
> >>
> >> 2. Process 'B' tries to acquire the mutex FS lock, which is not
> >> available. Hence, it goes to sleep. Due to the new plugging mechanism,
> >> before going to sleep, shcedule() is invoked which disables preemption
> >> and the context becomes atomic. In schedule(), the newly added
> >> blk_flush_plug_list() is invoked which unplugs the block driver.
> >>
> >> 3) During unplug operation the block driver tries to acquire the mutex
> >> lock which fails, because the lock was held by process 'A'. Previous
> >> invocation of scheudle() in step 2 has already made the context as
> >> atomic, hence the error "Schedule while atomic" occured.
> > if blk_flush_plug_list() is called in schedule(), it will use
> > blk_run_queue_async
> > to unplug the queue. This runs in a workqueue. So how could this happen?
> >
> 
> The call stack goes as follows:
> 
> From schedule() it calls blk_schedule_flush_plug()  and
> blk_flush_plug_list() gets invoked.
> 
> In blk_flush_plug_list() queue_unplugged() does not get invoked. Hence
>  blk_run_queue_async is not called.
> Instead __elv_add_request() is invoked with ELEVATOR_INSERT_SORT_MERGE
> flag and the flag gets reassigned to ELEVATOR_INSERT_BACK.
> 	
> In ELEVATOR_INSERT_BACK, __blk_run_queue() gets invoked and calls request_fn().
This doesn't make sense. why the flag is changed from
ELEVATOR_INSERT_SORT_MERGE to ELEVATOR_INSERT_BACK?

can you post a full log? or did your driver have something special?


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