linux-kernel - Re: sysfs methods can race with ->remove

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Date:	Mon, 26 Jan 2015 09:19:50 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Bart Van Assche <bvanassche@....org>,
	James Bottomley <jbottomley@...allels.com>,
	Hannes Reinecke <hare@...e.de>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: sysfs methods can race with ->remove

On Thu, Jan 15, 2015 at 02:40:31PM -0500, Tejun Heo wrote:
> > If a method is registered by the driver, then the driver will
> > unregister it when the ->remove routine runs.  I don't know for
> > certain, but I would expect that the sysfs/kernfs core will make sure
> > that any existing method calls complete before unregister returns.  
> > This would prevent races.
> 
> Yes, attribute deletions are blocked till the on-going sysfs
> read/write operations are finished and further rw accesses are failed.

Btw, where do we do that?  I did a walk through the code starting
from device_del, but must have missed the obvious.

> > The sriov_numvfs_store method does have the same problem, and so does 
> > the reset_store method (by way of pci_reset_function -> 
> > pci_dev_save_and_disable -> pci_reset_notify).
> > 
> > Tejun, is my analysis correct?  How should we fix these races?
> 
> I'm not really following what the actual problem case is, so SCSI
> subsystem store methods are derefing dev->driver without synchronizing
> against detach events?  If that's the case, the solution would be
> synchronizing against attach/detach events?  Sorry if I'm being
> totally idiotic.  I'm having a bit of hard time jumping right in.  :)

No problem.  That's the basic situation we are talking about.  I have
a serie fixing some long standing issues in the device model integration
in SCSI, and pointed out a possible issue in that area.

So what is the proper lock to take to prevent ->remove from beeing
called while in such a method?  A mentioned about I tried to peel
through all the layers of the onion^H^H^H^H^Hdriver core, but so far
couldn't find anything obvious.
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