linux-kernel - Re: [PATCH 2/5] jump label: base patch

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Date:	Fri, 26 Mar 2010 17:30:42 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Jason Baron <jbaron@...hat.com>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, fweisbec@...il.com
Subject: Re: [PATCH 2/5] jump label: base patch

Jason Baron wrote:
> base patch to implement 'jump labeling'. Based on a new 'asm goto' inline
> assembly gcc mechanism, we can now branch to labels from an 'asm goto'
> statment. This allows us to create a 'no-op' fastpath, which can subsequently
> be patched with a jump to the slowpath code. This is useful for code which
> might be rarely used, but which we'd like to be able to call, if needed.
> Tracepoints are the current usecase that these are being implemented for.
> 
> Signed-off-by: Jason Baron <jbaron@...hat.com>
> ---
>  include/asm-generic/vmlinux.lds.h |   10 ++-
>  include/linux/jump_label.h        |   57 +++++++++++++
>  kernel/Makefile                   |    2 +-
>  kernel/jump_label.c               |  165 +++++++++++++++++++++++++++++++++++++
>  4 files changed, 232 insertions(+), 2 deletions(-)
>  create mode 100644 include/linux/jump_label.h
>  create mode 100644 kernel/jump_label.c
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 67e6520..83a469d 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -167,7 +167,8 @@
>  	BRANCH_PROFILE()						\
>  	TRACE_PRINTKS()							\
>  	FTRACE_EVENTS()							\
> -	TRACE_SYSCALLS()
> +	TRACE_SYSCALLS()						\
> +	JUMP_TABLE()							\

Just a minor style issue: no need to add '\' on the last line.

>  
>  /*
>   * Data section helpers
> @@ -206,6 +207,7 @@
>  		*(__vermagic)		/* Kernel version magic */	\
>  		*(__markers_strings)	/* Markers: strings */		\
>  		*(__tracepoints_strings)/* Tracepoints: strings */	\
> +		*(__jump_strings)/* Jump: strings */	\
>  	}								\
>  									\

Just a minor style issue: please align the tab.

[...]
> +static struct jump_label_entry *add_jump_label_entry(const char *name, int nr_entries, struct jump_entry *table)
> +{
> +	struct hlist_head *head;
> +	struct hlist_node *node;
> +	struct jump_label_entry *e;
> +	size_t name_len = strlen(name) + 1;
> +	u32 hash = jhash(name, name_len-1, 0);
> +
> +	head = &jump_label_table[hash & (JUMP_LABEL_TABLE_SIZE - 1)];
> +	hlist_for_each_entry(e, node, head, hlist) {
> +		if (!strcmp(name, e->name))
> +			return ERR_PTR(-EEXIST);
> +	}
> +	e = kmalloc(sizeof(struct jump_label_entry) + name_len, GFP_KERNEL);
> +	if (!e)
> +		return ERR_PTR(-ENOMEM);
> +	memcpy(&e->name[0], name, name_len);

Hmm, why don't you just have a pointer for e->name which points name?
Or, maybe you can find it easily from e->table[0].name (so you can save
some memory).

Thank you,


-- 
Masami Hiramatsu
e-mail: mhiramat@...hat.com
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