Messages in this thread | | | Date | Tue, 18 Nov 2014 10:10:22 +0100 | From | Jan Kara <> | Subject | Re: [PATCH] printk: drop logbuf_cpu volatile qualifier |
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On Fri 14-11-14 13:19:31, Alex Elder wrote: > 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 value (and not an out-of-date cached one).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. > > Reported-by: Pranith Kumar <bobby.prani@gmail.com> > Fix-suggested-by: Steven Rostedt <rostedt@goodmis.org> > Signed-off-by: Alex Elder <elder@linaro.org> > --- > Note!!! I am not able to stress test this patch. The patch looks good to me. You can add: Reviewed-by: Jan Kara <jack@suse.cz>
BTW, stress-testing this is tough. I never saw printk recursion happening in practice - only when I screwed up something in printk code when experimenting.
Honza
> kernel/printk/printk.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index ced2b84..fefc8d0 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1627,7 +1627,7 @@ 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 == SCHED_MESSAGE_LOGLEVEL) { > level = -1; > -- > 1.9.1 > -- Jan Kara <jack@suse.cz> SUSE Labs, CR
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