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authorAlex Elder <elder@linaro.org>2014-12-10 15:51:21 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-12-10 17:41:11 -0800
commitf099755d4c8523d72b45f13f02d3fc375d080e18 (patch)
treedfac8803ab6e1bea7b8fa031c9b2d21200d6cdc9 /kernel/printk
parenta39d4a857d4bb0a62d6655c0d69f7387fe1ad160 (diff)
downloadblackbird-op-linux-f099755d4c8523d72b45f13f02d3fc375d080e18.tar.gz
blackbird-op-linux-f099755d4c8523d72b45f13f02d3fc375d080e18.zip
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 Steven Rostedt concluded that not only was "volatile" not needed, neither was it required to use ACCESS_ONCE(). I offered an elaborate description that concluded Steven 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 So the value that is read is the "real" value (and not an out-of-date cached one).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@linaro.org> Reported-by: Pranith Kumar <bobby.prani@gmail.com> Suggested-by: Steven Rostedt <rostedt@goodmis.org> Reviewed-by: Jan Kara <jack@suse.cz> Cc: Petr Mladek <pmladek@suse.cz> Cc: Luis R. Rodriguez <mcgrof@suse.com> Cc: Joe Perches <joe@perches.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1b7092dbb590..218ea26d75b8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1624,7 +1624,7 @@ asmlinkage int vprintk_emit(int facility, int level,
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;
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