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

[Paul E. McKenney]

On Tue, Jan 23, 2018 at 03:59:26PM +0800, lianglihao@xxxxxxxxxx wrote:
> From: Heng Zhang <heng.z@xxxxxxxxxx>
> 
> This RCU implementation (PRCU) is based on a fast consensus protocol
> published in the following paper:
> 
> Fast Consensus Using Bounded Staleness for Scalable Read-mostly Synchronization.
> Haibo Chen, Heng Zhang, Ran Liu, Binyu Zang, and Haibing Guan.
> IEEE Transactions on Parallel and Distributed Systems (TPDS), 2016.
> https://dl.acm.org/citation.cfm?id=3024114.3024143
> 
> Signed-off-by: Heng Zhang <heng.z@xxxxxxxxxx>
> Signed-off-by: Lihao Liang <lianglihao@xxxxxxxxxx>

A few comments and questions interspersed.

							Thanx, Paul

> ---
>  include/linux/prcu.h |  37 +++++++++++++++
>  kernel/rcu/Makefile  |   2 +-
>  kernel/rcu/prcu.c    | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/core.c  |   2 +
>  4 files changed, 165 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/prcu.h
>  create mode 100644 kernel/rcu/prcu.c
> 
> diff --git a/include/linux/prcu.h b/include/linux/prcu.h
> new file mode 100644
> index 00000000..653b4633
> --- /dev/null
> +++ b/include/linux/prcu.h
> @@ -0,0 +1,37 @@
> +#ifndef __LINUX_PRCU_H
> +#define __LINUX_PRCU_H
> +
> +#include <linux/atomic.h>
> +#include <linux/mutex.h>
> +#include <linux/wait.h>
> +
> +#define CONFIG_PRCU
> +
> +struct prcu_local_struct {
> +	unsigned int locked;
> +	unsigned int online;
> +	unsigned long long version;
> +};
> +
> +struct prcu_struct {
> +	atomic64_t global_version;
> +	atomic_t active_ctr;
> +	struct mutex mtx;
> +	wait_queue_head_t wait_q;
> +};
> +
> +#ifdef CONFIG_PRCU
> +void prcu_read_lock(void);
> +void prcu_read_unlock(void);
> +void synchronize_prcu(void);
> +void prcu_note_context_switch(void);
> +
> +#else /* #ifdef CONFIG_PRCU */
> +
> +#define prcu_read_lock() do {} while (0)
> +#define prcu_read_unlock() do {} while (0)
> +#define synchronize_prcu() do {} while (0)
> +#define prcu_note_context_switch() do {} while (0)

If CONFIG_PRCU=n and some code is built that uses PRCU, shouldn't you
get a build error rather than an error-free but inoperative PRCU?

Of course, Peter's question about purpose of the patch set applies
here as well.

> +
> +#endif /* #ifdef CONFIG_PRCU */
> +#endif /* __LINUX_PRCU_H */
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 23803c7d..8791419c 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -2,7 +2,7 @@
>  # and is generally not a function of system call inputs.
>  KCOV_INSTRUMENT := n
> 
> -obj-y += update.o sync.o
> +obj-y += update.o sync.o prcu.o
>  obj-$(CONFIG_CLASSIC_SRCU) += srcu.o
>  obj-$(CONFIG_TREE_SRCU) += srcutree.o
>  obj-$(CONFIG_TINY_SRCU) += srcutiny.o
> diff --git a/kernel/rcu/prcu.c b/kernel/rcu/prcu.c
> new file mode 100644
> index 00000000..a00b9420
> --- /dev/null
> +++ b/kernel/rcu/prcu.c
> @@ -0,0 +1,125 @@
> +#include <linux/smp.h>
> +#include <linux/prcu.h>
> +#include <linux/percpu.h>
> +#include <linux/compiler.h>
> +#include <linux/sched.h>
> +
> +#include <asm/barrier.h>
> +
> +DEFINE_PER_CPU_SHARED_ALIGNED(struct prcu_local_struct, prcu_local);
> +
> +struct prcu_struct global_prcu = {
> +	.global_version = ATOMIC64_INIT(0),
> +	.active_ctr = ATOMIC_INIT(0),
> +	.mtx = __MUTEX_INITIALIZER(global_prcu.mtx),
> +	.wait_q = __WAIT_QUEUE_HEAD_INITIALIZER(global_prcu.wait_q)
> +};
> +struct prcu_struct *prcu = &global_prcu;
> +
> +static inline void prcu_report(struct prcu_local_struct *local)
> +{
> +	unsigned long long global_version;
> +	unsigned long long local_version;
> +
> +	global_version = atomic64_read(&prcu->global_version);
> +	local_version = local->version;
> +	if (global_version > local_version)
> +		cmpxchg(&local->version, local_version, global_version);
> +}
> +
> +void prcu_read_lock(void)
> +{
> +	struct prcu_local_struct *local;
> +
> +	local = get_cpu_ptr(&prcu_local);
> +	if (!local->online) {
> +		WRITE_ONCE(local->online, 1);
> +		smp_mb();
> +	}
> +
> +	local->locked++;
> +	put_cpu_ptr(&prcu_local);
> +}
> +EXPORT_SYMBOL(prcu_read_lock);
> +
> +void prcu_read_unlock(void)
> +{
> +	int locked;
> +	struct prcu_local_struct *local;
> +
> +	barrier();
> +	local = get_cpu_ptr(&prcu_local);
> +	locked = local->locked;
> +	if (locked) {
> +		local->locked--;
> +		if (locked == 1)
> +			prcu_report(local);

Is ordering important here?  It looks to me that the compiler could
rearrange some of the accesses within prcu_report() with the local->locked
decrement.  There appears to be some potential for load and store tearing,
though perhaps you have verified that your compiler avoids this on
the architecture that you are using.

> +		put_cpu_ptr(&prcu_local);
> +	} else {

Hmmm...  We get here if the RCU read-side critical section was preempted.
If none of them are preempted, ->active_ctr remains zero.

> +		put_cpu_ptr(&prcu_local);
> +		if (!atomic_dec_return(&prcu->active_ctr))
> +			wake_up(&prcu->wait_q);
> +	}
> +}
> +EXPORT_SYMBOL(prcu_read_unlock);
> +
> +static void prcu_handler(void *info)
> +{
> +	struct prcu_local_struct *local;
> +
> +	local = this_cpu_ptr(&prcu_local);
> +	if (!local->locked)
> +		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.

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
>