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Subject: Re: [PATCH RFC tip/core/rcu 4/5] 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,
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        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org
References: <20201117004017.GA7444@paulmck-ThinkPad-P72>
 <20201117004052.14758-4-paulmck@kernel.org>
 <f7a47e5f-095c-e4ba-ece0-83f2137fb381@codeaurora.org>
 <20201121001336.GN1437@paulmck-ThinkPad-P72>
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
Message-ID: <a54bc494-30cf-4e4e-f895-4803c66040c2@codeaurora.org>
Date:   Sun, 22 Nov 2020 19:57:26 +0530
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On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
> On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
>> Hi Paul,
>>
>> On 11/17/2020 6:10 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. ]
>>> 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 fed4a2d..e9bd6fb 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..b405811 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;
>>> @@ -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. */
>>> @@ -147,14 +148,19 @@ 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)))
>>
>> Should this be USHORT_CMP_LT ?
> 
> I believe that you are correct.  As is, it works but does needless
> grace periods.
> 
>>>    		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;
>>> +
>>>    	if (!READ_ONCE(ssp->srcu_gp_running)) {
>>> +		cookie = get_state_synchronize_srcu(ssp);
>>> +		if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>> +			WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>
>> I was thinking of a case which might break with this.
>>
>> Consider a scenario, where GP is in progress and kworker is right
>> before below point, after executing callbacks:
>>
>> void srcu_drive_gp(struct work_struct *wp) {
>>    <snip>
> 
> We updated ->srcu_idx up here, correct?  So it has bottom bit zero.
> 
>>    while (lh) {
>>    <cb execution loop>
>>    }
>>    >>> CURRENT EXECUTION POINT
> 
> Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
> due to an interrupt.
> 
Looking more, issue can happen, even when kworker is waiting for GP
completion @

swait_event_exclusive(ssp->srcu_wq, 
!READ_ONCE(ssp->srcu_lock_nesting[idx]));

Other process can call call_srcu() and skip srcu_idx_max update, as
ssp->srcu_gp_running is true.


Thanks
Neeraj

>>    WRITE_ONCE(ssp->srcu_gp_running, false);
>>
>>    if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>>      schedule_work(&ssp->srcu_work);
>> }
>>
>> Now, at this instance, srcu_gp_start_if_needed() runs and samples
>> srcu_gp_running and returns, without updating srcu_idx_max
>>
>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
>> {
>>    if (!READ_ONCE(ssp->srcu_gp_running)) returns true
>>    <snip>
>> }
> 
> This could happen in an interrupt handler, so with you thus far.
> 
>> kworker running srcu_drive_gp() resumes and returns without queueing a new
>> schedule_work(&ssp->srcu_work); for new GP?
>>
>> Prior to this patch, call_srcu() enqueues a cb before entering
>> srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
>> queuing, and schedule a work for the new GP, for this scenario.
>>
>>
>>   	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);
>>
>> So, should the "cookie" calculation and "srcu_idx_max" setting be moved
>> outside of ssp->srcu_gp_running check and maybe return the same cookie to
>> caller and use that as the returned cookie from
>> start_poll_synchronize_srcu() ?
>>
>> srcu_gp_start_if_needed()
>>    cookie = get_state_synchronize_srcu(ssp);
>>    if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
>>       WRITE_ONCE(ssp->srcu_idx_max, cookie);
>>    if (!READ_ONCE(ssp->srcu_gp_running)) {
>>    <snip>
>> }
> 
> I believe that you are quite correct, thank you!
> 
> But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
> in a timer handler.  The race window is quite narrow, so testing it might
> be a challenge...
> 
> This is what I end up with:
> 
> 	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);
> 		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);
> 		}
> 	}
> 
> Does that look plausible?

Looks good.

> 
> 							Thanx, Paul
> 

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