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

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Date:   Fri, 3 Sep 2021 18:09:09 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Martin Kaiser <martin@...ser.cx>,
        Michael Straube <straube.linux@...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 9/2/21 12:32, 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 interruptable 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@rowland.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.
> 

Hi, Greg!

I've retested this patch with lockdep enabled and I actually hit a 
deadlock. It's really my fault to forgot about lockdep while testing v4, 
I am sorry about the situation.

Actually, the disconnect here is not the problem, the problem was in 
original code. Changing mutex_lock_interruptible to mutex_lock just 
helped to discover it.


The log:

[  252.063305] WARNING: possible recursive locking detected
[  252.063642] 5.14.0+ #9 Tainted: G         C
[  252.063946] --------------------------------------------
[  252.064282] ip/335 is trying to acquire lock:
[  252.064560] ffff888009ebad28 (pmutex){+.+.}-{4:4}, at: 
usbctrl_vendorreq+0xc5/0x4a0 [r8188eu]
[  252.065168]
[  252.065168] but task is already holding lock:
[  252.065536] ffffffffc021b3b8 (pmutex){+.+.}-{4:4}, at: 
netdev_open+0x3a/0x5f [r8188eu]
[  252.066085]
[  252.066085] other info that might help us debug this:
[  252.066494]  Possible unsafe locking scenario:
[  252.066494]
[  252.066866]        CPU0
[  252.067025]        ----
[  252.067184]   lock(pmutex);
[  252.067367]   lock(pmutex);
[  252.067548]
[  252.067548]  *** DEADLOCK ***
[  252.067548]
[  252.067920]  May be due to missing lock nesting notation
[  252.067920]
[  252.068346] 2 locks held by ip/335:
[  252.068570]  #0: ffffffffbda94628 (rtnl_mutex){+.+.}-{4:4}, at: 
rtnetlink_rcv_msg+0x1e0/0x660
[  252.069115]  #1: ffffffffc021b3b8 (pmutex){+.+.}-{4:4}, at: 
netdev_open+0x3a/0x5f [r8188eu]
[  252.069690]
[  252.069690] stack backtrace:
[  252.069968] CPU: 1 PID: 335 Comm: ip Tainted: G         C 
5.14.0+ #9
[  252.071111] Call Trace:
[  252.071273]  dump_stack_lvl+0x45/0x59
[  252.071513]  __lock_acquire.cold+0x1fe/0x31b
[  252.072709]  lock_acquire+0x157/0x3c0
[  252.074445]  __mutex_lock+0xf6/0xc90
[  252.076294]  usbctrl_vendorreq+0xc5/0x4a0 [r8188eu]
[  252.076651]  usb_read8+0x68/0x8f [r8188eu]
[  252.076962]  ? usb_read16+0x8e/0x8e [r8188eu]
[  252.077287]  _rtw_read8+0x2d/0x32 [r8188eu]
[  252.077601]  HalPwrSeqCmdParsing+0x143/0x1de [r8188eu]
[  252.077979]  rtl8188eu_InitPowerOn+0x5a/0xe0 [r8188eu]
[  252.078352]  rtl8188eu_hal_init+0xe7/0x1008 [r8188eu]
[  252.078989]  rtw_hal_init+0x38/0xb5 [r8188eu]
[  252.079317]  _netdev_open+0x282/0x4db [r8188eu]
[  252.079653]  netdev_open+0x42/0x5f [r8188eu]


The problem was here before, but it was race condition, rather than a 
deadlock: netdev_open() locks the mutex, but internally calls usb_read8().

With previous code mutex_lock_interruptible() just fails and execution 
goes forward. It's not correct anyway... Fabio's patch helps to discover 
design bug :)


Again, I am so sorry for not enabling lockdep while testing this first 
time...





With regards,
Pavel Skripkin