netdev - Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section

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Date:   Fri, 12 Jul 2019 17:35:59 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Borislav Petkov <bp@...en8.de>, c0d1n61at3@...il.com,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
        kernel-hardening@...ts.openwall.com, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        neilb@...e.com, netdev@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Pavel Machek <pavel@....cz>, peterz@...radead.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section

On Fri, Jul 12, 2019 at 01:00:18PM -0400, Joel Fernandes (Google) wrote:
> The rcu/sync code was doing its own check whether we are in a reader
> section. With RCU consolidating flavors and the generic helper added in
> this series, this is no longer need. We can just use the generic helper
> and it results in a nice cleanup.
> 
> Cc: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Hi Oleg,
Slightly unrelated to the patch,
I tried hard to understand this comment below in percpu_down_read() but no dice.

I do understand how rcu sync and percpu rwsem works, however the comment
below didn't make much sense to me. For one, there's no readers_fast anymore
so I did not follow what readers_fast means. Could the comment be updated to
reflect latest changes?
Also could you help understand how is a writer not able to change
sem->state and count the per-cpu read counters at the same time as the
comment tries to say?

	/*
	 * We are in an RCU-sched read-side critical section, so the writer
	 * cannot both change sem->state from readers_fast and start checking
	 * counters while we are here. So if we see !sem->state, we know that
	 * the writer won't be checking until we're past the preempt_enable()
	 * and that once the synchronize_rcu() is done, the writer will see
	 * anything we did within this RCU-sched read-size critical section.
	 */

Also,
I guess we could get rid of all of the gp_ops struct stuff now that since all
the callbacks are the same now. I will post that as a follow-up patch to this
series.

thanks!

 - Joel


> ---
> Please note: Only build and boot tested this particular patch so far.
> 
>  include/linux/rcu_sync.h |  5 ++---
>  kernel/rcu/sync.c        | 22 ----------------------
>  2 files changed, 2 insertions(+), 25 deletions(-)
> 
> diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
> index 6fc53a1345b3..c954f1efc919 100644
> --- a/include/linux/rcu_sync.h
> +++ b/include/linux/rcu_sync.h
> @@ -39,9 +39,8 @@ extern void rcu_sync_lockdep_assert(struct rcu_sync *);
>   */
>  static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
>  {
> -#ifdef CONFIG_PROVE_RCU
> -	rcu_sync_lockdep_assert(rsp);
> -#endif
> +	RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),
> +			 "suspicious rcu_sync_is_idle() usage");
>  	return !rsp->gp_state; /* GP_IDLE */
>  }
>  
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index a8304d90573f..535e02601f56 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -10,37 +10,25 @@
>  #include <linux/rcu_sync.h>
>  #include <linux/sched.h>
>  
> -#ifdef CONFIG_PROVE_RCU
> -#define __INIT_HELD(func)	.held = func,
> -#else
> -#define __INIT_HELD(func)
> -#endif
> -
>  static const struct {
>  	void (*sync)(void);
>  	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
>  	void (*wait)(void);
> -#ifdef CONFIG_PROVE_RCU
> -	int  (*held)(void);
> -#endif
>  } gp_ops[] = {
>  	[RCU_SYNC] = {
>  		.sync = synchronize_rcu,
>  		.call = call_rcu,
>  		.wait = rcu_barrier,
> -		__INIT_HELD(rcu_read_lock_held)
>  	},
>  	[RCU_SCHED_SYNC] = {
>  		.sync = synchronize_rcu,
>  		.call = call_rcu,
>  		.wait = rcu_barrier,
> -		__INIT_HELD(rcu_read_lock_sched_held)
>  	},
>  	[RCU_BH_SYNC] = {
>  		.sync = synchronize_rcu,
>  		.call = call_rcu,
>  		.wait = rcu_barrier,
> -		__INIT_HELD(rcu_read_lock_bh_held)
>  	},
>  };
>  
> @@ -49,16 +37,6 @@ enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
>  
>  #define	rss_lock	gp_wait.lock
>  
> -#ifdef CONFIG_PROVE_RCU
> -void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
> -{
> -	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
> -			 "suspicious rcu_sync_is_idle() usage");
> -}
> -
> -EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
> -#endif
> -
>  /**
>   * rcu_sync_init() - Initialize an rcu_sync structure
>   * @rsp: Pointer to rcu_sync structure to be initialized
> -- 
> 2.22.0.510.g264f2c817a-goog
>