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Subject: Re: [block subsystem] Need help to prevent races on unexpected
 device removal
From: Maxim Levitsky <maximlevitsky@gmail.com>
To: linux-kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <1262546027.4555.17.camel@maxim-laptop>
References: <1262546027.4555.17.camel@maxim-laptop>
Content-Type: text/plain; charset="UTF-8"
Date: Mon, 04 Jan 2010 21:56:57 +0200
Message-ID: <1262635017.10523.1.camel@maxim-laptop>
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On Sun, 2010-01-03 at 21:13 +0200, Maxim Levitsky wrote: 
> During development of hotplug support for mtd translation layer I seems
> to be unable to figure a way to prevent following race:
> 
> First of all, a block device is registered. I attach a private structure
> to that device to save all internal information.
> 
> Then out of the blue (when user pulls off the card) I receive a request
> to remove the device.
> 
> In the function that handles such removal, I do:
> 
> del_gendisk(...
> blk_start_queue
> 
> stop thread that processes the requests
> 
> blk_cleanup_queue(old->rq);
> 
> 
> The problem is that I don't know where/when to free the private
> structure.
> 
> I though about adding a field to the structure, with name 'invalid', so
> that release will not attempt to go futher, but free the structure, but
> what happens if release is never called?
> In other words this will work as long as there is a user of the block
> device.
> 
> I thought then that I can detect that condition and free the structure
> in the removal function itself, but then I get a race with ->open
> running in same time, and mutex will not prevent it, I will have to
> release it somewhen, and then ->open will access a freed structure....
> 
> 
> Best regards,
> Maxim Levitsky
> 

Still don't know how to do that properly....

Best regards,
Maxim Levitsky

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