linux-kernel - Re: [PATCH v5] staging: r8188eu: Remove _enter/_exit_critical

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Date:   Sat, 04 Sep 2021 10:15:03 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Martin Kaiser <martin@...ser.cx>,
        Michael Straube <straube.linux@...il.com>,
        Pavel Skripkin <paskripkin@...il.com>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Aakash Hemadri <aakashhemadri123@...il.com>
Subject: Re: [PATCH v5] staging: r8188eu: Remove _enter/_exit_critical_mutex()

On Thursday, September 2, 2021 11:32:07 AM CEST Greg Kroah-Hartman wrote:
> On Sat, Aug 28, 2021 at 01:36:56PM +0200, Fabio M. De Francesco wrote:
> > Remove _enter_critical_mutex() and _exit_critical_mutex(). They are
> > unnecessary wrappers, respectively to mutex_lock_interruptible() and
> > to mutex_unlock(). They also have an odd interface that takes an unused
> > argument named pirqL of type unsigned long.
> > The original code enters the critical section if the mutex API is
> > interrupted while waiting to acquire the lock; therefore it could lead
> > to a race condition. Use mutex_lock() because it is uninterruptible and
> > so avoid that above-mentioned potential race condition.
> > 
> > Tested-by: Pavel Skripkin <paskripkin@...il.com>
> > Reviewed-by: Pavel Skripkin <paskripkin@...il.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> > ---
> > 
> > v5: Fix a typo in the subject line. Reported by Aakash Hemadri.
> > 
> > v4: Tested and reviewed by Pavel Skripkin. No changes to the code.
> > 
> > v3: Assume that the original authors don't expect that
> > mutex_lock_interruptible() can be really interrupted and then lead to 
> > a potential race condition. Furthermore, Greg Kroah-Hartman makes me
> > notice that "[] one almost never needs interruptible locks in a driver".
> > Therefore, replace the calls to mutex_lock_interruptible() with calls to
> > mutex_lock() since the latter is uninterruptible and avoid race
> > conditions without the necessity to handle -EINTR errors.
> 
> Based on a recent conversation on the linux-usb mailing list, perhaps I
> was wrong:
> 	https://lore.kernel.org/r/
20210829015825.GA297712@...land.harvard.edu
> 
> Can you check what happens with your change when you disconnect the
> device and these code paths are being called?  That is when you do want
> the lock interrupted.
> 
> Yes, the logic still seems wrong, but I don't want to see the code now
> just lock up entirely with this change as it is a change in how things
> work from today.
> 
> thanks,
> 
> greg k-h

Hi Greg,

I guess you've already read the responses from Pavel: He tested the code, 
again and again, and it works properly (connect/disconnect, "ip link show",  
and so on). Furthermore, this patch already has his "Tested-by:" and 
"Reviewed-by:" tags.

Pavel and I agree on the fact that this patch can be applied as-is, however 
we obviously know that it's only up to you.

In the meantime he found and fixed a bad design issue that was revealed by 
using my patch.

Please, let me know if there is anything else I can do.

Thanks,

Fabio