lkml.org 
[lkml]   [2013]   [Dec]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt: Document ACCESS_ONCE()
On Wed,  4 Dec 2013 14:46:59 -0800
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> The situations in which ACCESS_ONCE() is required are not well documented,
> so this commit adds some verbiage to memory-barriers.txt.

[...]

> + But please note that the compiler is also closely watching what you
> + do with the value after the ACCESS_ONCE(). For example, suppose you
> + do the following and MAX is a preprocessor macro with the value 1:
> +
> + for ((tmp = ACCESS_ONCE(a)) % MAX)
> + do_something_with(tmp);

That sure looks like it was meant to be "while" instead of "for"?

[...]

> + (*) The compiler is within its rights to reorder memory accesses unless
> + you tell it not to. For example, consider the following interaction
> + between process-level code and an interrupt handler:
> +
> + void process_level(void)
> + {
> + msg = get_message();
> + flag = true;
> + }
> +
> + void interrupt_handler(void)
> + {
> + if (flag)
> + process_message(msg);
> + }
> +
> + There is nothing to prevent the the compiler from transforming
> + process_level() to the following, in fact, this might well be a
> + win for single-threaded code:
> +
> + void process_level(void)
> + {
> + flag = true;
> + msg = get_message();
> + }
> +
> + If the interrupt occurs between these two statement, then
> + interrupt_handler() might be passed a garbled msg. Use ACCESS_ONCE()
> + to prevent this as follows:
> +
> + void process_level(void)
> + {
> + ACCESS_ONCE(msg) = get_message();
> + ACCESS_ONCE(flag) = true;
> + }
> +
> + void interrupt_handler(void)
> + {
> + if (ACCESS_ONCE(flag))
> + process_message(ACCESS_ONCE(msg));
> + }

Looking at this, I find myself wondering why you couldn't just put a
barrier() between the two statements in process_level()? ACCESS_ONCE()
seems like a heavy hammer to just avoid reordering of two assignments.
What am I missing, and what could be added here to keep the other folks as
dense as me from missing the same thing?

Thanks,

jon


\
 
 \ /
  Last update: 2013-12-05 21:41    [W:0.187 / U:0.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site