linux-kernel - Re: [PATCH] staging: r8188eu: remove _rtw_mutex

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Message-ID: <YTMhcMSqWCepQMMT@kroah.com>
Date:   Sat, 4 Sep 2021 09:34:08 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     Larry.Finger@...inger.net, phil@...lpotter.co.uk,
        straube.linux@...il.com, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>
Subject: Re: [PATCH] staging: r8188eu: remove _rtw_mutex_{init,free}

On Fri, Sep 03, 2021 at 10:33:56PM +0300, Pavel Skripkin wrote:
> While testing Fabio's patch I hit lockdep warning about possible
> deadlock:
> 
> [  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);
> 
> There is one problem with this warning: there is no pmutex in this
> driver, *BUT* all mutexes are initialized via private _rtw_mutex_init
> API, which had struct mutex *pmutex argument.
> 
> So, all mutexes in this driver had same name in lockdep map. Of course,
> lockdep will complain about any nested locking.
> 
> Fix it by open-coding _rtw_mutex_{init,free} wrappers, because we do not
> need them at all.
> 
> Cc: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Fixes: 15865124feed ("staging: r8188eu: introduce new core dir for RTL8188eu driver")

No need for "Fixes:" on this, as it's not a bug, it's just a badly
designed implementation.

thanks,

greg k-h