Memory Barriers for (Ubuntu) Linux (i686) Post: 302429949

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Special Forums UNIX and Linux Applications High Performance Computing Memory Barriers for (Ubuntu) Linux (i686) Post 302429949 by gorga on Wednesday 16th of June 2010 08:20:50 AM

06-16-2010

Registered User

Memory Barriers for (Ubuntu) Linux (i686)

Hi all,

(Hope this is the right forum for this question)

I have some multi-threaded C code (compiled with GCC 4.4.3) which accesses shared variables. Although I've marked those variables with volatile to guard against compiler re-ordering, I'm concerned that processor out-of-order execution may cause my code to fail, and I'm looking for a "low-cost" method of guaranteeing ordering is maintained in my code.

For example, I have something like...

Code:

memset(&task, 0, sizeof(task_t));/* null memory */
task.id.prefix = prefix_id;
task.id.instance = instance_id;
/* write-memorybarrier required here */
task.state = task_ready;

Where I need to ensure that the "task state" is only set to "task_ready" after the previous instructions have been committed. As the "task" is shared between threads, another thread seeing the state as "ready" may try to access its member variables, so it's vital that the tasks "prefix" and "instance" have been updated.

I know this is a common problem and mutexes and semaphores provide in-built memory barriers to address this problem but I'm trying to build a scalable application and I want to avoid their use if possible. I also know GCC provides built-in atomic operations but I see they involve locking the data-bus, and I've heard about system primitives like "smp_wmb()" but I'm not sure how to incorporate these into my "user-space" program as they are platform dependent.

Therefore can anyone provide pointers or advise on how best (in terms of scalability and speed) to guarantee ordering is maintained?

Thanks.

gorga

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LEARN ABOUT DEBIAN

ubuntu-dev-tools

ubuntu-dev-tools(5)						File Formats Manual					       ubuntu-dev-tools(5)

NAME

       ubuntu-dev-tools - Configuration for the ubuntu-dev-tools package.

DESCRIPTION

       The  ubuntu-dev-tools  package  is  similar in scope to the devscripts(1) package, providing a collection of scripts which may be of use to
       Ubuntu and Debian developers or others wishing to build Debian packages.

       Some of these scripts have options which may be configured  on  a  system-wide  and  per-user  basis.   These  options  are  configured	in
       devscripts.conf(5).

       All variables are described in the script's manpages. Package-wide variables begin with "UBUNTUTOOLS" and are listed below.

       Every script which reads the configuration files can be forced to ignore them by using the --no-conf command-line option.

ENVIRONMENT

       All ubuntu-dev-tools configuration variables can be set (and overridden) by setting them in the environment (unlike devscripts).

       In addition, several scripts use the following environment variables:

       UBUMAIL
	      Overrides  DEBEMAIL  and	DEBFULLNAME  when  the	target	is  clearly  Ubuntu.   Can  either  contain an e-mail address or Full Name
	      <email@example.org>.

       DEBEMAIL, DEBFULLNAME
	      As in devscripts(1).

PACKAGE-WIDE VARIABLES
       The currently recognised package-wide variables are:

       UBUNTUTOOLS_BUILDER
	      This specifies the preferred test-builder, one of pbuilder (default), sbuild, pbuilder-dist.

       UBUNTUTOOLS_DEBIAN_MIRROR
	      The preferred Debian archive mirror.  Should be of the form http://ftp.debian.org/debian (no trailing slash).  If not specified, the
	      master will be used.

       UBUNTUTOOLS_DEBSEC_MIRROR
	      The  preferred Debian security archive mirror.  Should be of the form http://security.debian.org (no trailing slash).  If not speci-
	      fied, the master will be used.

       UBUNTUTOOLS_UBUNTU_MIRROR
	      The preferred Ubuntu archive mirror.  Should be of the form http://archive.ubuntu.com/ubuntu (no trailing slash).  If not specified,
	      the master will be used.

       UBUNTUTOOLS_UBUNTU_PORTS_MIRROR
	      The preferred Ubuntu archive mirror.  Should be of the form http://ports.ubuntu.com (no trailing slash).	If not specified, the mas-
	      ter will be used.

       UBUNTUTOOLS_LPINSTANCE
	      The launchpad instance to communicate with. e.g. production (default) or staging.

       UBUNTUTOOLS_MIRROR_FALLBACK
	      Whether or not to fall-back to the master archive mirror.  This is usually the desired behaviour, as mirrors can	lag  the  masters.
	      If on a private network with only a local mirror, you may want to set this to no.  One of yes (default) or no.

       UBUNTUTOOLS_UPDATE_BUILDER
	      Whether or not to update the test-builder before each test build.  One of yes or no (default).

       UBUNTUTOOLS_WORKDIR
	      The directory to use for preparing source packages etc.  When unset, defaults to a directory in /tmp/ named after the script.

SEE ALSO

       devscripts(1), devscripts.conf(5)

AUTHORS

       This manpage was written by Stefano Rivera <stefanor@ubuntu.com>.

ubuntu-dev-tools						 December 19 2010					       ubuntu-dev-tools(5)