linux-kernel - RE: [PATCH net-next v2] misc: Add Renesas Synchronization Management Unit (SMU) support

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Date:   Fri, 12 Feb 2021 15:39:03 +0000
From:   Min Li <min.li.xe@...esas.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "derek.kiernan@...inx.com" <derek.kiernan@...inx.com>,
        "dragan.cvetic@...inx.com" <dragan.cvetic@...inx.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next v2] misc: Add Renesas Synchronization Management
 Unit (SMU) support

> > +
> > +	/* Only one open per device at a time */
> > +	if (!atomic_dec_and_test(&rsmu->open_count)) {
> > +		atomic_inc(&rsmu->open_count);
> > +		return -EBUSY;
> 
> This does not do what you think it does, and does not prevent multiple
> applications from talking to your device at the same time.
> 
> There is no need for this at all, as it does not work, sorry.  If multiple apps
> talk to your device, it's their fault, not the kernel's fault, that things go
> wrong.
> 
> And I thought that Arnd already told you to fix this?
> 

Hi Greg

Sorry for not replying to the list, I am new so not very familiar with the process.

Can you elaborate why it doesn't work? I kind of borrow the idea from xilinx_sdfec.c and I don't see why it doesn't work.

I mean if an application failed at opening the device, how can it proceed to talk the device without a file descriptor?

Thanks

Min