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Date:   Thu, 24 May 2018 11:02:57 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Martin Liu <liumartin@google.com>
cc:     gregkh@linuxfoundation.org, <linux-usb@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <jenhaochen@google.com>
Subject: Re: [RFC] driver core: don't hold dev's parent lock when using async
 probe
In-Reply-To: <20180524140021.GA214888@google.com>
Message-ID: <Pine.LNX.4.44L0.1805241100000.1871-100000@iolanthe.rowland.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Thu, 24 May 2018, Martin Liu wrote:

> On Tue, May 22, 2018 at 01:09:44PM -0400, Alan Stern wrote:
> > On Tue, 22 May 2018, martin_liu wrote:
> > 
> > > not sure if we still need 'bf74ad5bc417 ("[PATCH] Hold the
> > > device's parent's lock during probe and remove")' since it has
> > > been there over 10 years. If we still need it and hard to fix it
> > > , the simple way is to find a place not to allow USB subsystem
> > > drivers to have async probe capability. Any suggestion is welcome.
> > 
> > I don't think the "allows_async_probing" attribute is the best way to 
> > attack this.  Some other approach, like a special-purpose flag, might 
> > be better.
> > 
> > Yes, USB still needs to have parent's locks held during probing.  
> > Here's the reason.  A USB device can have multiple interfaces, each
> > bound to its own driver.  A driver may sometimes need to issue a reset,
> > but in USB there's no way to reset a single interface.  Only the entire
> > device can be reset, and of course this affects all the interfaces.  
> > Therefore a driver needs to acquire the device lock before it can issue
> > a reset.
> > 
> > The problem is that the driver's thread may already hold the device
> > lock.  During a normal probe sequence, for example, the interfaces get
> > probed by the hub driver while it owns the device lock.  But for probes
> > under other circumstances (for example, if the user writes to the
> > driver's "bind" attribute in sysfs), the device lock might not be held.
> > 
> > A driver cannot tell these two cases apart.  The only way to make it
> > work all the time is to have the caller _always_ hold the device lock
> > while the driver is probed (or the removed, for that matter).
> > 
> > Alan Stern
> 
> Thanks for the reply and more detail about the backgroud. I'd like to
> have a conclusion about it. Please kindly correct me if my understanding
> is wrong. Regarding to the "special-purpose flag", do you mean we could
> find a place in USB subsystem to have the flag set (not sure if it's
> easy to find it). Driver core would be base on the flag to decide if we
> need to hold the device's parent's lock.

Yes, except that the flag would not be in the USB subsystem.  It would 
be in the device, device_type, or bus_type structure, so that the 
driver core could access it.

Alan Stern