Re: [PATCH RFC 01/16] prcu: Add PRCU implementation

[Boqun Feng]

On Tue, Jan 30, 2018 at 05:34:03AM +0000, zhangheng (AC) wrote:
[...]
> >> > +static void prcu_handler(void *info) {
> >> > +	struct prcu_local_struct *local;
> >> > +
> >> > +	local = this_cpu_ptr(&prcu_local);
> >> > +	if (!local->locked)
> >
> >And I think a smp_mb() is needed here, because in the following case:
> >
> >	CPU 0				          CPU 1
> >	==================		==========================
> >	{X is initially 0}
> >
> >	WRITE_ONCE(X, 1);
> >
> >	                      prcu_read_unlock(void):
> >	                      if (locked) {
> >	  				              synchronize_prcu(void):
> >					                ...
> >					                <send IPI to CPU 0>
> >	local->locked--;
> >	                      # switch to IPI
> > WRITE_ONCE(local->version,....)
> >	  				            <read CPU 0 version to be latest>
> >					              <return>
> >
> >					              r1 = READ_ONCE(X);
> >
> >r1 could be 0, which breaks RCU guarantees.
> >
> 
> Thank you.
> As I know,
> it guarantees that the interrupt to be handled after all write instructions issued before have complete in x86 arch.
> So the smp_mb is meaningless in x86 arch.

Sure. x86 is TSO, and we are talking about reordering of two stores
here, and that can not happen on TSO.

> But I am not sure whether other archs guarantee this feature. If not, we do need a smp_mb here.
> 

I think most of the weak memory model don't have this gaurantee, so you
need a smp_mb() or use smp_store_release().

> >> > +		WRITE_ONCE(local->version, atomic64_read(&prcu->global_version));
> >> > +}
> >> > +
> >> > +void synchronize_prcu(void)
> >> > +{
> >> > +	int cpu;
> >> > +	cpumask_t cpus;
> >> > +	unsigned long long version;
> >> > +	struct prcu_local_struct *local;
> >> > +
> >> > +	version = atomic64_add_return(1, &prcu->global_version);
> >> > +	mutex_lock(&prcu->mtx);
> >> > +
> >> > +	local = get_cpu_ptr(&prcu_local);
> >> > +	local->version = version;
> >> > +	put_cpu_ptr(&prcu_local);
> >> > +
> >> > +	cpumask_clear(&cpus);
> >> > +	for_each_possible_cpu(cpu) {
> >> > +		local = per_cpu_ptr(&prcu_local, cpu);
> >> > +		if (!READ_ONCE(local->online))
> >> > +			continue;
> >> > +		if (READ_ONCE(local->version) < version) {
> >> 
> >> On 32-bit systems, given that ->version is long long, you might see 
> >> load tearing.  And on some 32-bit systems, the cmpxchg() in 
> >> prcu_hander() might not build.
> >> 
> >
> >/me curious about why an atomic64_t is used here for global version. I think maybe 32bit global version still suffices.
> >
> >Regards,
> >Boqun
> 
> Because the synchronization latency is low, it can have higher gp frequency.
> It seems that 32bit can only correctly work for several years if there are 20+ gps per second.
> 

Because PRCU doesn't handle gp number overflow? May I ask why this is
difficult? Currently RCU could tolerate counter wrap for grace period:

	https://lwn.net/Articles/652677/ (Details in "Parallelism facts of life")

Is there any subtle difference I'm missing?

Regards,
Boqun

> >
> >> Or is the idea that only prcu_handler() updates ->version?  But in 
> >> that case, you wouldn't need the READ_ONCE() above.  What am I missing here?
> >> 
> >> > +			smp_call_function_single(cpu, prcu_handler, NULL, 0);
> >> > +			cpumask_set_cpu(cpu, &cpus);
> >> > +		}
> >> > +	}
> >> > +
> >> > +	for_each_cpu(cpu, &cpus) {
> >> > +		local = per_cpu_ptr(&prcu_local, cpu);
> >> > +		while (READ_ONCE(local->version) < version)
> >> 
> >> This ->version read can also tear on some 32-bit systems, and this one 
> >> most definitely can race with the prcu_handler() above.  Does the 
> >> algorithm operate correctly in that case?  (It doesn't look that way 
> >> to me, but I might be missing something.) Or are 32-bit systems excluded?
> >> 
> >> > +			cpu_relax();
> >> > +	}
> >> 
> >> I might be missing something, but I believe we need a memory barrier 
> >> here on non-TSO systems.  Without that, couldn't we miss a preemption?
> >> 
> >> > +
> >> > +	if (atomic_read(&prcu->active_ctr))
> >> > +		wait_event(prcu->wait_q, !atomic_read(&prcu->active_ctr));
> >> > +
> >> > +	mutex_unlock(&prcu->mtx);
> >> > +}
> >> > +EXPORT_SYMBOL(synchronize_prcu);
> >> > +
> >> > +void prcu_note_context_switch(void) {
> >> > +	struct prcu_local_struct *local;
> >> > +
> >> > +	local = get_cpu_ptr(&prcu_local);
> >> > +	if (local->locked) {
> >> > +		atomic_add(local->locked, &prcu->active_ctr);
> >> > +		local->locked = 0;
> >> > +	}
> >> > +	local->online = 0;
> >> > +	prcu_report(local);
> >> > +	put_cpu_ptr(&prcu_local);
> >> > +}
> >> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 
> >> > 326d4f88..a308581b 100644
> >> > --- a/kernel/sched/core.c
> >> > +++ b/kernel/sched/core.c
> >> > @@ -15,6 +15,7 @@
> >> >  #include <linux/init_task.h>
> >> >  #include <linux/context_tracking.h>  #include 
> >> > <linux/rcupdate_wait.h>
> >> > +#include <linux/prcu.h>
> >> > 
> >> >  #include <linux/blkdev.h>
> >> >  #include <linux/kprobes.h>
> >> > @@ -3383,6 +3384,7 @@ static void __sched notrace __schedule(bool 
> >> > preempt)
> >> > 
> >> >  	local_irq_disable();
> >> >  	rcu_note_context_switch(preempt);
> >> > +	prcu_note_context_switch();
> >> > 
> >> >  	/*
> >> >  	 * Make sure that signal_pending_state()->signal_pending() below
> >> > --
> >> > 2.14.1.729.g59c0ea183
> >> > 
> >> 
> >

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