[PATCH 2/3] printk: Add %ptc to safely print a task's comm

From: John Stultz
Date: Mon May 16 2011 - 17:19:28 EST

Accessing task->comm requires proper locking. However in the past
access to current->comm could be done without locking. This
is no longer the case, so all comm access needs to be done
while holding the comm_lock.

In my attempt to clean up unprotected comm access, I've noticed
most comm access is done for printk output. To simplify correct
locking in these cases, I've introduced a new %ptc format,
which will print the corresponding task's comm.

Example use:
printk("%ptc: unaligned epc - sending SIGBUS.\n", current);

CC: Ted Ts'o <tytso@xxxxxxx>
CC: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
CC: David Rientjes <rientjes@xxxxxxxxxx>
CC: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: linux-mm@xxxxxxxxx
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
lib/vsprintf.c | 24 ++++++++++++++++++++++++
1 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index bc0ac6b..b7a9953 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -797,6 +797,23 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
return string(buf, end, uuid, spec);

+static noinline_for_stack
+char *task_comm_string(char *buf, char *end, void *addr,
+ struct printf_spec spec, const char *fmt)
+ struct task_struct *tsk = addr;
+ char *ret;
+ unsigned long flags;
+ spin_lock_irqsave(&tsk->comm_lock, flags);
+ ret = string(buf, end, tsk->comm, spec);
+ spin_unlock_irqrestore(&tsk->comm_lock, flags);
+ return ret;
int kptr_restrict = 1;

@@ -864,6 +881,12 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,

switch (*fmt) {
+ case 't':
+ switch (fmt[1]) {
+ case 'c':
+ return task_comm_string(buf, end, ptr, spec, fmt);
+ }
+ break;
case 'F':
case 'f':
ptr = dereference_function_descriptor(ptr);
@@ -1151,6 +1174,7 @@ qualifier:
* http://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-00
* %pU[bBlL] print a UUID/GUID in big or little endian using lower or upper
* case.
+ * %ptc outputs the task's comm name
* %n is ignored
* The return value is the number of characters which would

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