Re: [PATCHv2] powerpc: ABI change - add HWCAP/HWCAP2/platform info to TCB

[Steven Munroe <munroesj at linux dot vnet dot ibm dot com>]

On Wed, 2015-08-26 at 21:41 -0500, Steven Munroe wrote:
> On Wed, 2015-08-26 at 16:40 -0400, Carlos O'Donell wrote:
> > On 08/12/2015 03:56 PM, Carlos Eduardo Seo wrote:
> > > - versioned symbol to track this change.
> > > - no âsetâ API in ppc.h
> > > - no per-thread initialization
> > > - added platform number (derived from AT_PLATFORM string) for future implementation of __builtin_cpu_is() in gcc.
> > 
> > Getting there, probably two more iterations.
> > 
> > You have concurrency issues.
> > 
> Thanks to both Carlos'. I will have to careful about how I say this to
> both  ...
> 
I really should not work after 9:00PM ... :(

> I think the comments and documentation should be explicit with the
> following assumptions.
> 
> The initialization of the static variables __tcb_hwcap, __tcb_platform
> and __tcb_hwcap_init must occur before main() gets control and before
> any other threads can exist.
> 
Before Library init/ctors and main() get control for the main thread.

> The TCB fields hwcap and at_platform must be initialized from the
> corresponding static variables before: Library init/ctors get control
> for the main thread, and before any new thread gets control from clone.
> 
before: Library init/ctors and main() get control for the main thread,
and before any new thread gets control from clone.


> As these are aligned words/dwords the stores are effective atomic
> relaxed MO in the PowerISA. In addition the stores to the static
> variables (__tcb_hwcap, __tcb_platform) occur while only one thread
> exists and the kernel will enforce sequential consistency before the
> child of the clone syscall can execute any load from static variable
> __tcb_hwcap, or __tcb_platform. As to copy (load of __tcb_hwcap,
> __tcb_platform and store or TCB fields hwcap and at_platform) always
> occurs within a single thread and copies to thread local storage, there
> is no data race.
> 
> Does this help?
> 
Hopefully this helps even more.