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Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for
 Tiny SRCU grace periods
To:     paulmck@kernel.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        kent.overstreet@gmail.com
References: <@@@> <20201121005919.17152-4-paulmck@kernel.org>
 <2d210fa7-3484-9cee-862b-a8f15fee8c8e@codeaurora.org>
 <20201123211250.GF1437@paulmck-ThinkPad-P72>
 <9ebe7c82-6fc2-a759-dde8-eec90b483735@codeaurora.org>
 <20201124193043.GK1437@paulmck-ThinkPad-P72>
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
Message-ID: <229a3481-8d3e-6c81-0c67-b7f6c17a2291@codeaurora.org>
Date:   Wed, 25 Nov 2020 10:09:22 +0530
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On 11/25/2020 1:00 AM, Paul E. McKenney wrote:
> On Tue, Nov 24, 2020 at 10:44:24AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
>>> On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
>>>>
>>>>
>>>> On 11/21/2020 6:29 AM, paulmck@kernel.org wrote:
>>>>> From: "Paul E. McKenney" <paulmck@kernel.org>
>>>>>
>>>>> There is a need for a polling interface for SRCU grace
>>>>> periods, so this commit supplies get_state_synchronize_srcu(),
>>>>> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
>>>>> purpose.  The first can be used if future grace periods are inevitable
>>>>> (perhaps due to a later call_srcu() invocation), the second if future
>>>>> grace periods might not otherwise happen, and the third to check if a
>>>>> grace period has elapsed since the corresponding call to either of the
>>>>> first two.
>>>>>
>>>>> As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
>>>>> the return value from either get_state_synchronize_srcu() or
>>>>> start_poll_synchronize_srcu() must be passed in to a later call to
>>>>> poll_state_synchronize_srcu().
>>>>>
>>>>> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
>>>>> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
>>>>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
>>>>> [ paulmck: Apply feedback from Neeraj Upadhyay. ]
>>>>> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
>>>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>>>> ---
>>>>>     include/linux/rcupdate.h |  2 ++
>>>>>     include/linux/srcu.h     |  3 +++
>>>>>     include/linux/srcutiny.h |  1 +
>>>>>     kernel/rcu/srcutiny.c    | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
>>>>>     4 files changed, 56 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>>>> index de08264..e09c0d8 100644
>>>>> --- a/include/linux/rcupdate.h
>>>>> +++ b/include/linux/rcupdate.h
>>>>> @@ -33,6 +33,8 @@
>>>>>     #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
>>>>>     #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
>>>>>     #define ulong2long(a)		(*(long *)(&(a)))
>>>>> +#define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
>>>>> +#define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
>>>>>     /* Exported common interfaces */
>>>>>     void call_rcu(struct rcu_head *head, rcu_callback_t func);
>>>>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
>>>>> index e432cc9..a0895bb 100644
>>>>> --- a/include/linux/srcu.h
>>>>> +++ b/include/linux/srcu.h
>>>>> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
>>>>>     int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
>>>>>     void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
>>>>>     void synchronize_srcu(struct srcu_struct *ssp);
>>>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
>>>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
>>>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
>>>>>     #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
>>>>> index d9edb67..c7f0c1f 100644
>>>>> --- a/include/linux/srcutiny.h
>>>>> +++ b/include/linux/srcutiny.h
>>>>> @@ -16,6 +16,7 @@
>>>>>     struct srcu_struct {
>>>>>     	short srcu_lock_nesting[2];	/* srcu_read_lock() nesting depth. */
>>>>>     	unsigned short srcu_idx;	/* Current reader array element in bit 0x2. */
>>>>> +	unsigned short srcu_idx_max;	/* Furthest future srcu_idx request. */
>>>>>     	u8 srcu_gp_running;		/* GP workqueue running? */
>>>>>     	u8 srcu_gp_waiting;		/* GP waiting for readers? */
>>>>>     	struct swait_queue_head srcu_wq;
>>>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>>>>> index 3bac1db..b073175 100644
>>>>> --- a/kernel/rcu/srcutiny.c
>>>>> +++ b/kernel/rcu/srcutiny.c
>>>>> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
>>>>>     	ssp->srcu_gp_running = false;
>>>>>     	ssp->srcu_gp_waiting = false;
>>>>>     	ssp->srcu_idx = 0;
>>>>> +	ssp->srcu_idx_max = 0;
>>>>>     	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
>>>>>     	INIT_LIST_HEAD(&ssp->srcu_work.entry);
>>>>>     	return 0;
>>>>
>>>> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
>>>>
>>>> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
>>>> WARN_ON(ssp->srcu_idx & 1);
>>>
>>> Good point, added and under test.
>>>
>>>> Thanks
>>>> Neeraj
>>>>
>>>>> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
>>>>>     	struct srcu_struct *ssp;
>>>>>     	ssp = container_of(wp, struct srcu_struct, srcu_work);
>>>>> -	if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
>>>>> +	if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>>>     		return; /* Already running or nothing to do. */
>>>>>     	/* Remove recently arrived callbacks and wait for readers. */
>>>>> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp)
>>>>>     	 * straighten that out.
>>>>>     	 */
>>>>>     	WRITE_ONCE(ssp->srcu_gp_running, false);
>>>>> -	if (READ_ONCE(ssp->srcu_cb_head))
>>>>> +	if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>>>>     		schedule_work(&ssp->srcu_work);
>>>>>     }
>>>>>     EXPORT_SYMBOL_GPL(srcu_drive_gp);
>>>>>     static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>>>>     {
>>>>> +	unsigned short cookie;
>>>>> +
>>>>> +	cookie = get_state_synchronize_srcu(ssp);
>>>>> +	if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>>>> +		WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>>>
>>>> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
>>>> scheduling work?
>>>
>>> How about like this?
>>
>> Looks good!
> 
> Are you willing to give an ack or reviewed-by for either patch?
> 
> 							Thanx, Paul
> 

For the version in rcu -dev

Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>

Thanks
Neeraj

>> Thanks
>> Neeraj
>>
>>>
>>> 	static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>>> 	{
>>> 		unsigned short cookie;
>>>
>>> 		cookie = get_state_synchronize_srcu(ssp);
>>> 		if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> 			return;
>>> 		WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>> 		if (!READ_ONCE(ssp->srcu_gp_running)) {
>>> 			if (likely(srcu_init_done))
>>> 				schedule_work(&ssp->srcu_work);
>>> 			else if (list_empty(&ssp->srcu_work.entry))
>>> 				list_add(&ssp->srcu_work.entry, &srcu_boot_list);
>>> 		}
>>> 	}
>>>
>>> Testing this next.  ;-)
>>>
>>> 							Thanx, Paul
>>>
>>>> Thanks
>>>> Neeraj
>>>>
>>>>>     	if (!READ_ONCE(ssp->srcu_gp_running)) {
>>>>>     		if (likely(srcu_init_done))
>>>>>     			schedule_work(&ssp->srcu_work);
>>>>> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
>>>>>     }
>>>>>     EXPORT_SYMBOL_GPL(synchronize_srcu);
>>>>> +/*
>>>>> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
>>>>> + */
>>>>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
>>>>> +{
>>>>> +	unsigned long ret;
>>>>> +
>>>>> +	barrier();
>>>>> +	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
>>>>> +	barrier();
>>>>> +	return ret & USHRT_MAX;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
>>>>> +
>>>>> +/*
>>>>> + * start_poll_synchronize_srcu - Provide cookie and start grace period
>>>>> + *
>>>>> + * The difference between this and get_state_synchronize_srcu() is that
>>>>> + * this function ensures that the poll_state_synchronize_srcu() will
>>>>> + * eventually return the value true.
>>>>> + */
>>>>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
>>>>> +{
>>>>> +	unsigned long ret = get_state_synchronize_srcu(ssp);
>>>>> +
>>>>> +	srcu_gp_start_if_needed(ssp);
>>>>> +	return ret;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>>>>> +
>>>>> +/*
>>>>> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
>>>>> + */
>>>>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>>>>> +{
>>>>> +	bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
>>>>> +
>>>>> +	barrier();
>>>>> +	return ret;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
>>>>> +
>>>>>     /* Lockdep diagnostics.  */
>>>>>     void __init rcu_scheduler_starting(void)
>>>>>     {
>>>>>
>>>>
>>>> -- 
>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>>>> the Code Aurora Forum, hosted by The Linux Foundation
>>
>> -- 
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>> the Code Aurora Forum, hosted by The Linux Foundation

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation