Re: [PATCH] Allow times and time system calls to return smallnegative values

From: Ingo Molnar
Date: Thu Dec 04 2008 - 10:42:29 EST

* Paul Mackerras <paulus@xxxxxxxxx> wrote:

> Ingo Molnar writes:
> > > + force_successful_syscall_return();
> > > return compat_jiffies_to_clock_t(jiffies);
> >
> > just curious: what code does force_successful_syscall_return() actually
> > run in the powerpc case - those bits are missing from this patch. I
> > suspect it sets some sort of flag?
> It sets the TIF_NOERROR thread flag, which is tested in the syscall
> exit path along with various other thread flags.
> force_successful_syscall_return() is defined for powerpc in
> arch/powerpc/include/asm/ptrace.h. It's nothing new, it has existed
> for ages and is used in a few other places already. It has existing
> non-null definitions on alpha, ia64, powerpc, and sparc64.

ok, was just curious. To make this code nicer, we could keep the time
syscalls from returning small negative numbers.

the core issue is probably:

jiffies.h:#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))

the thing is, we did INITIAL_JIFFIES to find _in kernel_ jiffies
wraparoudn bugs (hung timers, broken drivers, etc.) - we did not want to
confuse userspace with it.

So how about adding INITIAL_JIFFIES to the sys_times() return value?

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