linux-kernel - Re: [PATCH] net/mlx4: Avoid 'may be used uninitialized' warnings

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Message-Id: <20150514.223012.1808412142637659689.davem@davemloft.net>
Date:	Thu, 14 May 2015 22:30:12 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	bhelgaas@...gle.com
Cc:	ogerlitz@...lanox.com, jackm@....mellanox.co.il,
	matanb@...lanox.com, idos@...lanox.com, netdev@...r.kernel.org,
	yishaih@...lanox.com, amirv@...lanox.com,
	linux-kernel@...r.kernel.org, roland@...estorage.com
Subject: Re: [PATCH] net/mlx4: Avoid 'may be used uninitialized' warnings

From: Bjorn Helgaas <bhelgaas@...gle.com>
Date: Thu, 14 May 2015 18:17:08 -0500

> With a cross-compiler based on gcc-4.9, I see warnings like the following:
> 
>   drivers/net/ethernet/mellanox/mlx4/resource_tracker.c: In function 'mlx4_SW2HW_CQ_wrapper':
>   drivers/net/ethernet/mellanox/mlx4/resource_tracker.c:3048:10: error: 'cq' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>     cq->mtt = mtt;
> 
> I think the warning is spurious because we only use cq when
> cq_res_start_move_to() returns zero, and it always initializes *cq in that
> case.  The srq case is similar.  But maybe gcc isn't smart enough to figure
> that out.
> 
> Initialize cq and srq explicitly to avoid the warnings.
> 
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>

Applied.

The compiler, generally, is not good at determining use-before-initialized
in situations of the form:

	int x;

	if (foo)
		x = whatever;
	...
	if (foo)
		use(x);
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