linux-kernel - Re: [PATCH 1/4] input: Introduce buflock, a one-to-many circular buffer mechanism

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Message-ID: <4C092BA7.507@bitmath.org>
Date:	Fri, 04 Jun 2010 18:36:55 +0200
From:	Henrik Rydberg <rydberg@...math.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Kosina <jkosina@...e.cz>,
	Mika Kuoppala <mika.kuoppala@...ia.com>,
	Benjamin Tissoires <tissoire@...a.fr>,
	Rafi Rubin <rafi@...s.upenn.edu>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/4] input: Introduce buflock, a one-to-many circular
 buffer mechanism

>> +#define buflock_write(bw, buf, size, item)				\
>> +	do {								\
>> +		bw.next_head = (bw.head + 1) & ((size) - 1);		\
>> +		smp_wmb();						\
> 
> Why do we need the write barrier here?

I believe my first answer to this question was foggy indeed, so allow me to go
again, with a time line:

Scenario 1, correct write order:

writer              store_next_head   store_buf        store_head
reader  load_head   load_buf       load_next_head

Result: head != next_head, incoherent read detected

Scenario 2, incorrect write order:

writer              store_buf         store_next_head  store_head
reader  load_head   load_buf       load_next_head

Result: head == next_head, incoherent read not detected

Based on the assumption that scenario 2 could happen if the smp_wmb() is not
present, the barrier is needed.

Henrik
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