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

Denys Vlasenko
2008-09-10T09:31:05Z

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

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

Updated patch is below.

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);