Re: [PATCH] make setpriority POSIX compliant; introduce PRIO_THREAD extension

Michael Kerrisk
2008-09-10T11:57:46Z

Denys,

On 9/10/08, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> On Tue, 2008-09-09 at 17:45 +0200, Michael Kerrisk wrote:
> > > Patch is run tested. I will post test program etc as a reply.
> >
> > Tested-by: Michael Kerrisk <mtk.manpages@gmail.com>
> >
> > Please do CC me on API changes, so that they might get documented in
> > the man pages.
> >
> > Thanks for this work. (I'm not sure whether or not it's a response to
> > my bug report, http://bugzilla.kernel.org/show_bug.cgi?id=6258 )
>
> I worked on it because of this bug report:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=455251

(That report is inaccessible, although I have some kind of bugzilla
login there: "You are not authorized to access bug #455251.")

> > I tested your patch. Most things seem to work as I would expect, but
> > there is one strangeness.
> >
> > I would expect
> > setpriority(PRIO_PROCESS, getpid())
> > and
> > setpriority(PRIO_PROCESS, 0)
> > to have the same affect (because: which == PRIO_PRCESS, who == 0
> > conventionally means "the calling process").
> >
> > But they do not: the latter call only changes the priority of the
> > calling thread. Is this intended?
>
> No, it was not intended. I think this error is here:
>
> + case PRIO_PROCESS:
> + if (who)
> + pid = find_vpid(who);
> + else {
> + pid = task_pid(current);
> + who = current->pid;
> + }
>
> I was confused. ->pid is TID, had to use ->tgid to get PID.

(Indeed as I read Chris Freiesen's mail I almost tripped up with the
same confusion. The source code names of these fields can easily lead
to confusions like that.)

> The fix: replace
> who = current->pid;
> with
> who = current->tgid;
> There are two places where you need to do it.
>
> Updated patch is below.

Tested-by: Michael Kerrisk <mtk.manpages@gmail.com>

With your suggested change, things now work as I would expect. Thanks!

Cheers,

Michael

> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> --
> vda
>
>
>
> diff --git a/include/linux/resource.h b/include/linux/resource.h
> index aaa423a..f292690 100644
> --- a/include/linux/resource.h
> +++ b/include/linux/resource.h
> @@ -51,6 +51,7 @@ struct rlimit {
> #define PRIO_PROCESS 0
> #define PRIO_PGRP 1
> #define PRIO_USER 2
> +#define PRIO_THREAD 3
>
> /*
> * Limit the stack by to some sane default: root can always
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 038a7bc..d339c1a 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -142,9 +142,9 @@ asmlinkage long sys_setpriority(int which, int who,
> int niceval)
> struct task_struct *g, *p;
> struct user_struct *user;
> int error = -EINVAL;
> - struct pid *pgrp;
> + struct pid *pgrp, *pid;
>
> - if (which > PRIO_USER || which < PRIO_PROCESS)
> + if (which > PRIO_THREAD || which < PRIO_PROCESS)
> goto out;
>
> /* normalize: avoid signed division (rounding problems) */
> @@ -156,7 +156,7 @@ asmlinkage long sys_setpriority(int which, int who,
> int niceval)
>
> read_lock(&tasklist_lock);
> switch (which) {
> - case PRIO_PROCESS:
> + case PRIO_THREAD:
> if (who)
> p = find_task_by_vpid(who);
> else
> @@ -164,6 +164,19 @@ asmlinkage long sys_setpriority(int which, int who,
> int niceval)
> if (p)
> error = set_one_prio(p, niceval, error);
> break;
> + case PRIO_PROCESS:
> + if (who)
> + pid = find_vpid(who);
> + else {
> + pid = task_pid(current);
> + who = current->tgid;
> + }
> + do_each_pid_thread(pid, PIDTYPE_PID, p) {
> + if (who == p->pid || who == p->tgid) {
> + error = set_one_prio(p, niceval, error);
> + }
> + } while_each_pid_thread(pid, PIDTYPE_PID, p);
> + break;
> case PRIO_PGRP:
> if (who)
> pgrp = find_vpid(who);
> @@ -206,14 +219,14 @@ asmlinkage long sys_getpriority(int which, int
> who)
> struct task_struct *g, *p;
> struct user_struct *user;
> long niceval, retval = -ESRCH;
> - struct pid *pgrp;
> + struct pid *pgrp, *pid;
>
> - if (which > PRIO_USER || which < PRIO_PROCESS)
> + if (which > PRIO_THREAD || which < PRIO_PROCESS)
> return -EINVAL;
>
> read_lock(&tasklist_lock);
> switch (which) {
> - case PRIO_PROCESS:
> + case PRIO_THREAD:
> if (who)
> p = find_task_by_vpid(who);
> else
> @@ -224,6 +237,21 @@ asmlinkage long sys_getpriority(int which, int who)
> retval = niceval;
> }
> break;
> + case PRIO_PROCESS:
> + if (who)
> + pid = find_vpid(who);
> + else {
> + pid = task_pid(current);
> + who = current->tgid;
> + }
> + do_each_pid_thread(pid, PIDTYPE_PID, p) {
> + if (who == p->pid || who == p->tgid) {
> + niceval = 20 - task_nice(p);
> + if (niceval > retval)
> + retval = niceval;
> + }
> + } while_each_pid_thread(pid, PIDTYPE_PID, p);
> + break;
> case PRIO_PGRP:
> if (who)
> pgrp = find_vpid(who);
>
>
>
>

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html