linux-kernel - Re: [PATCH v2 1/2] kernel: printk: specify alignment for struct printk

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Date:	Fri, 14 Nov 2014 09:22:08 -0800
From:	Joe Perches <joe@...ches.com>
To:	Andrey Ryabinin <a.ryabinin@...sung.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Jonathan Corbet <corbet@....net>,
	Michal Marek <mmarek@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Yury Gribov <y.gribov@...sung.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Konstantin Khlebnikov <koct9i@...il.com>, x86@...nel.org,
	linux-doc@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH v2 1/2] kernel: printk: specify alignment for struct
 printk_log

On Fri, 2014-11-14 at 15:50 +0300, Andrey Ryabinin wrote:
> On architectures that have support for efficient unaligned access
> struct printk_log has 4-byte alignment.
> Specify alignment attribute in type declaration.
> 
> The whole point of this patch is to fix deadlock which happening
> when UBSan detects unaligned access in printk() thus UBSan recursively
> calls printk() with logbuf_lock held by top printk() call.
[]
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
[]
> @@ -223,7 +223,11 @@ struct printk_log {
>  	u8 facility;		/* syslog facility */
>  	u8 flags:5;		/* internal record flags */
>  	u8 level:3;		/* syslog level */
> -};
> +}
> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> +__packed __aligned(4)
> +#endif

Why is adding __packed useful or __aligned(4) useful?

The struct is naturally aligned on u64 and should be
the size of 2 u64s.

struct printk_log {
	u64 ts_nsec;		/* timestamp in nanoseconds */
	u16 len;		/* length of entire record */
	u16 text_len;		/* length of text buffer */
	u16 dict_len;		/* length of dictionary buffer */
	u8 facility;		/* syslog facility */
	u8 flags:5;		/* internal record flags */
	u8 level:3;		/* syslog level */
};

Is there any case when it's not sizeof(u64) * 2?


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