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Message-Id: <20140110155952.b376106ab7b76403af99111b@linux-foundation.org>
Date: Fri, 10 Jan 2014 15:59:52 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: pavel@....cz, joe@...ches.com, keescook@...omium.org,
geert@...ux-m68k.org, jkosina@...e.cz, viro@...iv.linux.org.uk,
davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf: add %pT format specifier
On Thu, 9 Jan 2014 21:52:00 +0900 Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
> Hello.
>
> Since addition of %pT itself seems to be agreed,
sort-of. The reason I suggested inventing a new token was code
density: avoid pointlessly passing current all the time.
Oh well, whatever - this patch has other intentions.
> I refreshed this patch using
> linux-next-20140109. Please pick up if this patch is OK for you; I will start
> making patches for killing most of direct ->comm readers.
>
> Regards.
> ----------------------------------------
> >From 0d1f03d59a477459f3d3c190593d9e78f5d67de8 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Thu, 9 Jan 2014 21:32:22 +0900
> Subject: [PATCH] lib/vsprintf: add %pT format specifier
>
> Since task_struct->comm can be modified by other threads while the current
> thread is reading it, it is recommended to use get_task_comm() for reading it.
>
> However, since get_task_comm() holds task_struct->alloc_lock spinlock,
> some users cannot use get_task_comm(). Also, a lot of users are directly
> reading from task_struct->comm even if they can use get_task_comm().
> Such users might obtain inconsistent result.
>
> This patch introduces %pT format specifier for printing task_struct->comm.
> Currently %pT does not provide consistency. I'm planning to change to use RCU
> in the future. By using RCU, the comm name read from task_struct->comm will be
> guaranteed to be consistent.
Not completely accurate - RCU won't protect code which accesses ->comm
from interrupts. Printing current->comm from irq is quite daft, but I
bet there's code that does it :(
As long as the kernel doesn't crash or otherwise misbehave when this
happens, I think we're OK.
(And I guess there's also non-daft code which accesses current->comm
from interrupt context: oops, panic, etc).
> But before modifying set_task_comm() to use RCU,
> we need to kill direct ->comm users who do not use get_task_comm().
>
> An example for converting direct ->comm users is shown below. Since many debug
> printings use p == current, you can pass NULL instead of p if p == current.
>
> pr_info("comm=%s\n", p->comm); => pr_info("comm=%pT\n", p);
> pr_info("comm=%s\n", current->comm); => pr_info("comm=%pT\n", NULL);
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Reviewed-by: Pavel Machek <pavel@....cz>
> ---
> Documentation/printk-formats.txt | 6 ++++++
> lib/vsprintf.c | 20 +++++++++++++++++++-
> 2 files changed, 25 insertions(+), 1 deletions(-)
>
> diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
> index 6f4eb32..94459b4 100644
> --- a/Documentation/printk-formats.txt
> +++ b/Documentation/printk-formats.txt
> @@ -184,6 +184,12 @@ dentry names:
> equivalent of %s dentry->d_name.name we used to use, %pd<n> prints
> n last components. %pD does the same thing for struct file.
>
> +task_struct comm name:
> +
> + %pT
> +
> + For printing task_struct->comm.
> +
> struct va_format:
>
> %pV
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 185b6d3..a97f18b 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1179,6 +1179,21 @@ char *address_val(char *buf, char *end, const void *addr,
> return number(buf, end, num, spec);
> }
>
> +static noinline_for_stack
hm, does noinline_for_stack actually do anything useful here? I
suspect it *increases* stack depth in the way comm_name() is used here.
> +char *comm_name(char *buf, char *end, struct task_struct *tsk,
> + struct printf_spec spec, const char *fmt)
> +{
> + char name[TASK_COMM_LEN];
> +
> + /* Caller can pass NULL instead of current. */
> + if (!tsk)
> + tsk = current;
> + /* Not using get_task_comm() in case I'm in IRQ context. */
> + memcpy(name, tsk->comm, TASK_COMM_LEN);
> + name[sizeof(name) - 1] = '\0';
get_task_comm() uses strncpy()?
> + return string(buf, end, name, spec);
> +}
> +
--
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