linux-kernel - Re: [PATCH] staging: pi433: add descriptions for mutex locks

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Date:   Fri, 23 Mar 2018 19:00:27 +0100
From:   Valentin Vidic <Valentin.Vidic@...Net.hr>
To:     Marcus Wolf <marcus.wolf@...rthome-wolf.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Simon Sandström <simon@...anor.nu>,
        Marcus Wolf <linux@...f-Entwicklungen.de>,
        Luca Söthe <luca@...l.me>,
        Marcin Ciupak <marcin.s.ciupak@...il.com>,
        Michael Panzlaff <michael.panzlaff@....de>,
        Derek Robson <robsonde@...il.com>, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: pi433: add descriptions for mutex locks

On Fri, Mar 23, 2018 at 04:38:50PM +0100, Marcus Wolf wrote:
> I had no time to work on the code for monthes now and the memorisation
> of my thoughts when I was programming that (approx. one year ago) is
> quite pale.
> 
> As far as I remember, I read something, that the fifo has an integrated
> protection, so no external protection is needed. But absolutely unsure.
> 
> If I will find some time within the next days, I'll have a look at the
> code and try to recall.
> 
> But the most important thing already took place: We started thinking
> about it :-)

You are right, here is what kfifo.h says:

/*
 * Note about locking : There is no locking required until only * one reader
 * and one writer is using the fifo and no kfifo_reset() will be * called
 *  kfifo_reset_out() can be safely used, until it will be only called
 * in the reader thread.
 *  For multiple writer and one reader there is only a need to lock the writer.
 * And vice versa for only one writer and multiple reader there is only a need
 * to lock the reader.
 */

In the case of pi433 there is only one reader (pi433_tx_thread) and
there is no need for a lock there. But the char device (pi433_write)
might have multiple writers so we leave the mutex just in that function?

-- 
Valentin