linux-kernel - Re: [PATCH] USB: core: hub: fix usb_hub worker blocking drain_all

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Message-ID: <Y+HObaGgp3fwrL/7@rowland.harvard.edu>
Date:   Mon, 6 Feb 2023 23:07:09 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Zhongjie Zhu <zhongjiezhu1@...il.com>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: core: hub: fix usb_hub worker blocking
 drain_all_pages() worker issue

On Tue, Feb 07, 2023 at 10:02:51AM +0800, Zhongjie Zhu wrote:
> On Mon, Feb 6, 2023 at 11:17 PM Alan Stern <stern@...land.harvard.edu> wrote:
> >
> > On Mon, Feb 06, 2023 at 11:33:15AM +0800, 朱忠杰 wrote:
> > > Yes, this is a very special case.
> > >
> > > It will happen only when disconnecting the mass storage if there are
> > > too many files in the storage, and the scanning operation is running,
> > > and the file system is not unmounted.
> > > It looks like this issue should be fixed in the usb mass storage
> > > driver, but I don't find an appropriate place.
> >
> > That's not surprising, because usb-storage doesn't know anything about
> > what's happening on the mass-storage device it connects to.  All it does
> > is send the commands that it gets from the SCSI subsystem to the device
> > and receive the results back.  It has no idea whether there is a mounted
> > filesystem on the device, if the filesystem contains any files, or
> > whether a scanning operation is running,
> >
> > A better place to look for fixing this might be the filesystem code.
> > That's where the information about mounting, files, and scanning can be
> > found.
> >
> > Alan Stern
> 
> The problem is there is a for loop in the invalidate_inodes(), this
> function is in the block device driver. when the usb_disconnect is
> called, the filesystem is not umounted, userspace applications will be
> noticed the usb storage is disconnected, and then do the umounting
> work.
> the invalidate_inodes() is called in the usb hub worker, and will run
> for a long time. To fix this issue, the long running loop need to be
> moved out from the usb hub worker.

Oh, maybe I didn't understand.

You've got a USB mass-storage device with a mounted filesystem and a lot 
of dirty inodes, right?  Then a USB disconnect happens, and as part of 
the disconnect processing, invalidate_inodes() runs for a long time.

Do you know why it takes so long?  The I/O operations shouldn't need any 
time; they will all fail immediately because the device has been 
disconnected and so there is no way to communicate with it.

Alan Stern