Re: [PATCH] x86/mce: Distirbute the clear operation of mces_seen to Per-CPU rather than only monarch CPU

[Chen Yucong]

On Mon, 2014-05-19 at 09:26 +0200, Borislav Petkov wrote:
> On Mon, May 19, 2014 at 09:55:40AM +0800, Chen Yucong wrote:
> > But all other CPUs also have to wait monarch CPU to exit from mce_end.
> > What's the difference between monarch CPU and Per-CPU for clearing
> > mces_seen? In practice, there is no difference between them. If we use
> > monarch CPU to clear mces_seen, then Per-CPU variable can not play out
> > its advantage.
> 
> I'll let you stare at mce_reign() a little bit longer... Also, pay
> attention to its callsite, that might help.
> 
We can find the following code segment in mce_end:
-----
...
        if (order == 1) {
                /* CHECKME: Can this race with a parallel hotplug? */
                int cpus = num_online_cpus();

                /*
                 * Monarch: Wait for everyone to go through their
scanning
                 * loops.
                 */
                while (atomic_read(&mce_executing) <= cpus) {
                        if (mce_timed_out(&timeout))
                                goto reset;
                        ndelay(SPINUNIT);
                }

                mce_reign();
                barrier();
                ret = 0;
...
-----
If a timeout occurs in monarch CPU, what will happen for the above code
segment?
The monarch CPU will directly execute -goto reset-, so mce_reign will
not be invoked. That way, the clear operation of mces_seen will be
skipped, and the stale value of mces_seen will reappear on the next mce.

thx!
cyc


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