The patch titled
Subject: printk: drop logbuf_cpu volatile qualifier
has been added to the -mm tree. Its filename is
printk-drop-logbuf_cpu-volatile-qualifier.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/printk-drop-logbuf_cpu-volatile-qualifier.patch
echo and later at
echo http://ozlabs.org/~akpm/mmotm/broken-out/printk-drop-logbuf_cpu-volatile-qualifier.patch
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------------------------------------------------------
From: Alex Elder <elder@xxxxxxxxxx>
Subject: printk: drop logbuf_cpu volatile qualifier
Pranith Kumar posted a patch in which removed the "volatile"
qualifier for the "logbuf_cpu" variable in vprintk_emit().
https://lkml.org/lkml/2014/11/13/894
In his patch, he used ACCESS_ONCE() for all references to
that symbol to provide whatever protection was intended.
There was some discussion that followed, and in the end
Stephen Rostedt concluded that not only was "volatile" not
needed, neither was it required to use ACCESS_ONCE(). I
offered an elaborate description that concluded Stephen
was right, and Pranith asked me to submit an alternative
patch. And this is it.
The basic reason "volatile" is not needed is that "logbuf_cpu" has
static storage duration, and vprintk_emit() is an exported
interface. This means that the value of logbuf_cpu must be read
from memory the first time it is used in a particular call of
vprintk_emit(). The variable's value is read only once in that
function, when it's read it'll be the copy from memory (or cache).
In addition, the value of "logbuf_cpu" is only ever written under
protection of a spinlock. So the value that is read is the "real"
value (and not an out-of-date cached one). If its value is not
UINT_MAX, it is the current CPU's processor id, and it will have
been last written by the running CPU.
Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
Reported-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
Suggested-by: Stephen Rostet <rostedt@xxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxx>
Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN kernel/printk/printk.c~printk-drop-logbuf_cpu-volatile-qualifier kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-drop-logbuf_cpu-volatile-qualifier
+++ a/kernel/printk/printk.c
@@ -1624,7 +1624,7 @@ asmlinkage int vprintk_emit(int facility
int printed_len = 0;
bool in_sched = false;
/* cpu currently holding logbuf_lock in this function */
- static volatile unsigned int logbuf_cpu = UINT_MAX;
+ static unsigned int logbuf_cpu = UINT_MAX;
if (level == LOGLEVEL_SCHED) {
level = LOGLEVEL_DEFAULT;
_
Patches currently in -mm which might be from elder@xxxxxxxxxx are
printk-drop-logbuf_cpu-volatile-qualifier.patch
linux-next.patch
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