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Date: Mon, 6 Jan 2014 23:00:56 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: joe@...ches.com
Cc: keescook@...omium.org, pavel@....cz, akpm@...ux-foundation.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[C012] format specifier
Joe Perches wrote:
> On Sun, 2014-01-05 at 12:15 +0900, Tetsuo Handa wrote:
> > > Since my purpose is to make reading of task_struct->comm consistent, %pT-like
> > > extension is what I want for centralizing pointer dereferences.
> >
> > If we have no objections for %pT[C012] patch,
>
> I still believe emitting different output styles using
> %pT[012] is not ideal.
>
> Is this really necessary?
No problem. %pT[012] are simply optimization (reducing number of function
arguments for saving text size) and therefore I can drop them.
What about below patch?
----------
>>From f69a1db69a9ad6e14235edbb5a9b7d720530a2be Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Mon, 6 Jan 2014 22:22:00 +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. 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>
---
Documentation/printk-formats.txt | 6 ++++++
lib/vsprintf.c | 20 +++++++++++++++++++-
2 files changed, 25 insertions(+), 1 deletion(-)
diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 445ad74..bd96e34 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -177,6 +177,12 @@ 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 10909c5..0300dae 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1155,6 +1155,21 @@ char *netdev_feature_string(char *buf, char *end, const u8 *addr,
return number(buf, end, *(const netdev_features_t *)addr, spec);
}
+static noinline_for_stack
+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';
+ return string(buf, end, name, spec);
+}
+
int kptr_restrict __read_mostly;
/*
@@ -1221,6 +1236,7 @@ int kptr_restrict __read_mostly;
* - 'a' For a phys_addr_t type and its derivative types (passed by reference)
* - 'd[234]' For a dentry name (optionally 2-4 last components)
* - 'D[234]' Same as 'd' but for a struct file
+ * - 'T' task_struct->comm
*
* Note: The difference between 'S' and 'F' is that on ia64 and ppc64
* function pointers are really function descriptors, which contain a
@@ -1232,7 +1248,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
{
int default_width = 2 * sizeof(void *) + (spec.flags & SPECIAL ? 2 : 0);
- if (!ptr && *fmt != 'K') {
+ if (!ptr && *fmt != 'K' && *fmt != 'T') {
/*
* Print (null) with the same width as a pointer so it makes
* tabular output look nice.
@@ -1364,6 +1380,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
return dentry_name(buf, end,
((const struct file *)ptr)->f_path.dentry,
spec, fmt);
+ case 'T':
+ return comm_name(buf, end, ptr, spec, fmt);
}
spec.flags |= SMALL;
if (spec.field_width == -1) {
--
1.7.9.5
--
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