linux-kernel - Re: [linux-usb-devel] [PATCH] base/class.c: prevent ooops due to insert/remove race (v3)

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Date:	Thu, 29 Nov 2007 21:58:20 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Mark Lord <lkml@....ca>, Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<linux-usb-devel@...ts.sourceforge.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [linux-usb-devel] [PATCH] base/class.c: prevent ooops due to
 insert/remove race (v3)

On Thu, 29 Nov 2007, Linus Torvalds wrote:

> Heh. It definitely hasn't gotten lost by "the git software".

No, it sure hasn't.  In fact it was staring me right in the face and I 
didn't realize it.

> In fact, with 
> the kinds of hints you already gave, git makes it really _trivial_ to find 
> it.
> 
> Here's what you do:
> 
> 	git log v2.6.23.. --author=Wilcox
> 
> and then just search for "scan_mutex", in the hope that Matthew wrote a 
> nice commit message. And yes, he did, so in less than a blink you get:
> 
> 	commit 6b7f123f378743d739377871c0cbfbaf28c7d25a
> 	Author: Matthew Wilcox <matthew@....cx>
> 	Date:   Tue Jun 26 15:18:51 2007 -0600
> 	
> 	    [SCSI] Fix async scanning double-add problems
> 
> 	    Stress-testing and some thought has revealed some places where
> 	    asynchronous scanning needs some more attention to locking.
> 	
> 	     - Since async_scan is a bit, we need to hold the host_lock while
> 	       modifying it to prevent races against other CPUs modifying the word
> 	       that bit is in.  This is probably a theoretical race for the moment,
> 	       but other patches may change that.
> 	     - The async_scan bit means not only that this host is being scanned
> 	       asynchronously, but that all the devices attached to this host are not
> 	       yet added to sysfs.  So we must ensure that this bit is always in sync.
> 	       I've chosen to do this with the scan_mutex since it's already acquired
> 	       in most of the right places.
> 		...
> 
> which I assume is the commit you're talking about.

Yep, that's the one.

Alan Stern

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