* boot time variable
@ 2017-10-09 23:17 Tobin C. Harding
2017-10-10 7:08 ` Greg KH
2017-10-10 7:50 ` valdis.kletnieks at vt.edu
0 siblings, 2 replies; 4+ messages in thread
From: Tobin C. Harding @ 2017-10-09 23:17 UTC (permalink / raw)
To: kernelnewbies
Hi,
I would like to create a boot time variable i.e a variable that is set once at boot time. Variable
does not need to be globally accessible. (actually I am using two variables).
Could any one point me to examples of this already intree please?
I have tried the following but it has a race condition on the zero check and assignment of randval/oddval.
/* Maps a pointer to a unique identifier. */
static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
{
long hashval;
static long randval = 0;
static long oddval = 0;
if (oddval == 0 && randval == 0) {
randval = get_random_long();
oddval = get_random_odd_long();
}
hashval = ptr_obfuscate((unsigned long)ptr, randval, oddval);
spec.base = 16;
return number(buf, end, hashval, spec);
}
And the compiler doesn't like
static long randval = get_random_long();
static long oddval = get_random_odd_long();
I thought of wrapping oddval/randval in a struct and protecting it with a lock but I don't know
how/where to initialize the lock in a race free manner?
Any tips or pointers please?
thanks,
Tobin.
^ permalink raw reply [flat|nested] 4+ messages in thread
* boot time variable
2017-10-09 23:17 boot time variable Tobin C. Harding
@ 2017-10-10 7:08 ` Greg KH
2017-10-10 10:33 ` Tobin C. Harding
2017-10-10 7:50 ` valdis.kletnieks at vt.edu
1 sibling, 1 reply; 4+ messages in thread
From: Greg KH @ 2017-10-10 7:08 UTC (permalink / raw)
To: kernelnewbies
On Tue, Oct 10, 2017 at 10:17:09AM +1100, Tobin C. Harding wrote:
> Hi,
>
> I would like to create a boot time variable i.e a variable that is set once at boot time. Variable
> does not need to be globally accessible. (actually I am using two variables).
static foo = 42;
should be all you need, right?
If not, what exactly do you mean by "boot time variable"?
> Could any one point me to examples of this already intree please?
>
> I have tried the following but it has a race condition on the zero check and assignment of randval/oddval.
>
> /* Maps a pointer to a unique identifier. */
> static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> {
> long hashval;
> static long randval = 0;
> static long oddval = 0;
>
> if (oddval == 0 && randval == 0) {
> randval = get_random_long();
> oddval = get_random_odd_long();
> }
>
> hashval = ptr_obfuscate((unsigned long)ptr, randval, oddval);
> spec.base = 16;
>
> return number(buf, end, hashval, spec);
> }
What's wrong with this code?
> And the compiler doesn't like
>
> static long randval = get_random_long();
> static long oddval = get_random_odd_long();
Yeah, that will not work, static initializers are at link/load time, not
runtime.
> I thought of wrapping oddval/randval in a struct and protecting it with a lock but I don't know
> how/where to initialize the lock in a race free manner?
Put a local lock in the function when testing if the variables are == 0,
if you are worried that two different callers will enter it at the same
time.
hope this helps,
greg k-h
^ permalink raw reply [flat|nested] 4+ messages in thread
* boot time variable
2017-10-09 23:17 boot time variable Tobin C. Harding
2017-10-10 7:08 ` Greg KH
@ 2017-10-10 7:50 ` valdis.kletnieks at vt.edu
1 sibling, 0 replies; 4+ messages in thread
From: valdis.kletnieks at vt.edu @ 2017-10-10 7:50 UTC (permalink / raw)
To: kernelnewbies
On Tue, 10 Oct 2017 10:17:09 +1100, "Tobin C. Harding" said:
> I would like to create a boot time variable i.e a variable that is set once at boot time. Variable
> does not need to be globally accessible. (actually I am using two variables).
The canonical way to have stuff happen at boot is usually declaring
an initcall.
> how/where to initialize the lock in a race free manner?
Step 0: Figure out how once-at-boot code could *possibly* be called
a second time. What are you worried about racing here?
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^ permalink raw reply [flat|nested] 4+ messages in thread
* boot time variable
2017-10-10 7:08 ` Greg KH
@ 2017-10-10 10:33 ` Tobin C. Harding
0 siblings, 0 replies; 4+ messages in thread
From: Tobin C. Harding @ 2017-10-10 10:33 UTC (permalink / raw)
To: kernelnewbies
On Tue, Oct 10, 2017 at 09:08:30AM +0200, Greg KH wrote:
> On Tue, Oct 10, 2017 at 10:17:09AM +1100, Tobin C. Harding wrote:
> > Hi,
> >
> > I would like to create a boot time variable i.e a variable that is set once at boot time. Variable
> > does not need to be globally accessible. (actually I am using two variables).
>
> static foo = 42;
>
> should be all you need, right?
>
> If not, what exactly do you mean by "boot time variable"?
>
> > Could any one point me to examples of this already intree please?
> >
> > I have tried the following but it has a race condition on the zero check and assignment of randval/oddval.
> >
> > /* Maps a pointer to a unique identifier. */
> > static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> > {
> > long hashval;
> > static long randval = 0;
> > static long oddval = 0;
> >
> > if (oddval == 0 && randval == 0) {
> > randval = get_random_long();
> > oddval = get_random_odd_long();
> > }
> >
> > hashval = ptr_obfuscate((unsigned long)ptr, randval, oddval);
> > spec.base = 16;
> >
> > return number(buf, end, hashval, spec);
> > }
>
> What's wrong with this code?
Maybe I just have race conditions on the brain.
> > And the compiler doesn't like
> >
> > static long randval = get_random_long();
> > static long oddval = get_random_odd_long();
>
> Yeah, that will not work, static initializers are at link/load time, not
> runtime.
>
> > I thought of wrapping oddval/randval in a struct and protecting it with a lock but I don't know
> > how/where to initialize the lock in a race free manner?
>
> Put a local lock in the function when testing if the variables are == 0,
> if you are worried that two different callers will enter it at the same
> time.
Okay.
> hope this helps,
>
> greg k-h
Cheers Greg
^ permalink raw reply [flat|nested] 4+ messages in thread
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2017-10-09 23:17 boot time variable Tobin C. Harding
2017-10-10 7:08 ` Greg KH
2017-10-10 10:33 ` Tobin C. Harding
2017-10-10 7:50 ` valdis.kletnieks at vt.edu
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