netdev - Re: [PATCH v3 2/6] PM / Runtime: introduce pm_runtime_set[get]_memalloc

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Date:	Tue, 30 Oct 2012 12:54:33 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ming Lei <ming.lei@...onical.com>
cc:	linux-kernel@...r.kernel.org, Oliver Neukum <oneukum@...e.de>,
	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Jens Axboe <axboe@...nel.dk>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH v3 2/6] PM / Runtime: introduce pm_runtime_set[get]_memalloc_noio()

On Wed, 31 Oct 2012, Ming Lei wrote:

> On Tue, Oct 30, 2012 at 11:38 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> 
> >
> > Okay, I see your point.  But acquiring the lock here doesn't solve the
> > problem.  Suppose a thread is about to reset a USB mass-storage device.
> > It acquires the lock and sees that the noio flag is clear.  But before
> > it can issue the reset, another thread sets the noio flag.
> 
> If the USB mass-storage device is being reseted, the flag should be set
> already generally.  If the flag is still unset, that means the disk/network
> device isn't added into system(or removed just now), so memory allocation
> with block I/O should be allowed during the reset. Looks it isn't one problem,
> isn't it?

As Oliver said, it can be a problem.

> > Lastly, pm_runtime_get_memalloc_noio always returns false when
> > CONFIG_PM_RUNTIME is disabled.  But we still need to prevent I/O during
> > usb_reset_device even when there's no runtime PM.  Maybe the simplest
> > answer is always to set noio during resets.  That would also help with
> > the race described above.
> 
> I have thought about this. IMO, pm_runtime_get_memalloc_noio should
> return true always if CONFIG_PM_RUNTIME is unset.

That's okay as long as the only user of pm_runtime_get_memalloc_noio
(apart from the runtime PM core) is usbcore.

Alan Stern

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