linux-kernel - Re: [linux-pm] [PATCH] Remove process freezer from suspend to RAM pathway

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Message-ID: <Pine.LNX.4.44L0.0707041030540.25704-100000@netrider.rowland.org>
Date:	Wed, 4 Jul 2007 10:38:47 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Matthew Garrett <mjg59@...f.ucam.org>
cc:	linux-kernel@...r.kernel.org, <linux-pm@...ts.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH] Remove process freezer from suspend to RAM
 pathway

On Tue, 3 Jul 2007, Matthew Garrett wrote:

> On Tue, Jul 03, 2007 at 06:21:42PM -0400, Alan Stern wrote:
> > On Tue, 3 Jul 2007, Matthew Garrett wrote:
> > > We're used to the idea of applications blocking when a resource they're 
> > > using goes away - NFS has done it forever. 
> > 
> > You persist in evading my point.  I'm not worried about applications;  
> > I'm worried about drivers.
> > 
> > Let me put it explicitly: You're writing a driver.  You're working on
> > the read, write, or probe method.  You add code to check if a system
> > sleep is underway.  Suppose the answer is Yes -- what does your driver
> > do next?
> 
> Leave the process blocked and defer any i/o until after resume. Why does 
> it need to be any more complicated than that?

(1) The driver will undoubtedly hold some mutex or semaphore at the 
time it checks whether a system sleep is underway.  You will have to 
drop it before blocking and then reacquire it afterward.

(2) The driver may have been called by some other routine which holds a 
mutex needed for resuming the device.  In this case the driver _can't_ 
drop the mutex and so the resume will deadlock.

Okay, I agree that (1) can be handled without too much effort.  But 
doing it adds an extra test to _every_ driver's I/O pathway.  Freezing 
userspace does not incur all this additional overhead.

(2) shouldn't arise during normal read and write operations, but it
certainly _will_ arise during probe.

Alan Stern

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