linux-kernel - Re: [PATCH] LDT - Linux Driver Template

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Message-ID: <CAE7jHC-Rbz0zA7UAAzs8iX9mHAC0bLUZY-igM8k7R258_sE0mA@mail.gmail.com>
Date:	Wed, 14 Nov 2012 15:04:42 +0200
From:	Constantine Shulyupin <const@...elinux.com>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LDT - Linux Driver Template

On Wed, Nov 14, 2012 at 5:42 AM, Ryan Mallon <rmallon@...il.com> wrote:
> On 14/11/12 05:46, Constantine Shulyupin wrote:
>> From: Constantine Shulyupin <const@...eLinux.com>
>> +     if (kfifo_is_empty(&in_fifo)) {
>
> Doesn't this require locking against whatever is filling the fifo?
I seems doesn't.
Note from include/linux/kfifo.h
/*
 * 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.
 */

>> +     if (mutex_lock_interruptible(&read_lock)) {
> The read_lock is only used here, so would only protect against
> concurrent reads. Isn't the read for a device function already
> synchronised against itself by the caller?
I can't find locking for read/write in sys_read, vfs_read
I just found lock misc_mtx for misc_open, misc_register.

>> +     ret = kfifo_from_user(&out_fifo, buf, count, &copied);
> Shouldn't this function be grabbing fifo_lock to prevent concurrent
> access to the fifo by the tasklet function? write_lock has the same
> issues described for read_lock above.

Accordingly note about kfifo locking is shouldn't.
kfifo supports asynchronous i/o.

Thank you.
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