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SubjectRe: [patch 11/17] Implement immediate update via stop_machine_run
* KOSAKI Motohiro (kosaki.motohiro@jp.fujitsu.com) wrote:
> Hi
>
> > -Updating immediate values, cannot rely on smp_call_function() b/c synchronizing
> > cpus using IPIs leads to deadlocks. Process A held a read lock on
> > tasklist_lock, then process B called apply_imv_update(). Process A received the
> > IPI and begins executing ipi_busy_loop(). Then process C takes a write lock
> > irq on the task list lock, before receiving the IPI. Thus, process A holds up
> > process C, and C can't get an IPI b/c interrupts are disabled. Solve this
> > problem by using a new 'ALL_CPUS' parameter to stop_machine_run(). Which
> > runs a function on all cpus after they are busy looping and have disabled
> > irqs. Since this is done in a new process context, we don't have to worry
> > about interrupted spin_locks. Also, less lines of code. Has survived 24 hours+
> > of testing...
>
> it seems this patch is must, Why do you separate patch [10/17] and [11/17]?
> this patch remove almost portion of [10/17].
> IMHO these patch merge into 1 patch is better.
>

Hi Kosaki,

You are right, I will merge them and resend them in the following post.

>
> > +static int stop_machine_imv_update(void *imv_ptr)
> > +{
> > + struct __imv *imv = imv_ptr;
> > +
> > + if (!wrote_text) {
>
> it seems racy.
> Why don't need test_and_set?
>
> I think your stop_machine_run(ALL_CPUS) call fn concurrency...
>
The answer to this mistery is in include/linux/stop_machine.h modified
by add-all-cpus-option-to-stop-machine-run.patch :


/**
* stop_machine_run: freeze the machine on all CPUs and run this function
* @fn: the function to run
* @data: the data ptr for the @fn()
- * @cpu: the cpu to run @fn() on (or any, if @cpu == NR_CPUS.
+ * @cpu: if @cpu == n, run @fn() on cpu n
+ * if @cpu == NR_CPUS, run @fn() on any cpu
+ * if @cpu == ALL_CPUS, run @fn() first on the calling cpu, and then
+ * concurrently on all the other cpus
*
* Description: This causes a thread to be scheduled on every other cpu,
* each of which disables interrupts, and finally interrupts are disabled

Therefore, the first execution of the function is done before all other
executions.

Thanks,

Mathieu

>
> > + text_poke((void *)imv->imv, (void *)imv->var, imv->size);
> > + wrote_text = 1;
> > + smp_wmb(); /* make sure other cpus see that this has run */
> > + } else
> > + sync_core();
> > +
> > + flush_icache_range(imv->imv, imv->imv + imv->size);
> > +
> > + return 0;
> > +}
> > +
>
>

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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