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List:       linux-usb-devel
Subject:    Re: [linux-usb-devel] Re: USB ioctl problem in 2.4.28
From:       Alan Stern <stern () rowland ! harvard ! edu>
Date:       2004-11-30 16:40:03
Message-ID: Pine.LNX.4.44L0.0411301136490.1189-100000 () ida ! rowland ! org
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On Tue, 30 Nov 2004, Meelis Roos wrote:

> > I do not see a good solution at this time.
> >
> > rwsem is a non-starter. IMHO, rwsems are optimizations for normal semaphores,
> > and the code must remain correct regardless what is used.
> 
> Why is an additional lock needed at all? Why can't we just use devsem?

Because of the need to synchronize with the usb-storage driver.  devsem 
exists only within the usbfs code.

> Also, what exactly is the correctness issue here?

As Oliver explained, some USB mass storage devices will crash if they 
receive a control message while carrying out an I/O operation.

Alan Stern



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