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Subject: Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit
 grace-period counter
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
To:     paulmck@kernel.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
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        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
        kent.overstreet@gmail.com
References: <@@@> <20201121005919.17152-1-paulmck@kernel.org>
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Message-ID: <28a22477-9fbd-603a-404c-1cf6e7cc18b5@codeaurora.org>
Date:   Wed, 25 Nov 2020 10:03:26 +0530
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On 11/24/2020 10:48 AM, Neeraj Upadhyay wrote:
> 
> 
> On 11/24/2020 1:25 AM, Paul E. McKenney wrote:
>> On Mon, Nov 23, 2020 at 10:01:13AM +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.  This
>>>> polling needs to distinguish between an SRCU instance being idle on the
>>>> one hand or in the middle of a grace period on the other.  This commit
>>>> therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from
>>>> a defacto boolean to a free-running counter, using the bottom bit to
>>>> indicate that a grace period is in progress.  The second-from-bottom
>>>> bit is thus used as the index returned by srcu_read_lock().
>>>>
>>>> Link: 
>>>> https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
>>>> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
>>>> [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per 
>>>> Upadhyay. ]
>>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>>> ---
>>>>    include/linux/srcutiny.h | 4 ++--
>>>>    kernel/rcu/srcutiny.c    | 5 +++--
>>>>    2 files changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
>>>> index 5a5a194..d9edb67 100644
>>>> --- a/include/linux/srcutiny.h
>>>> +++ b/include/linux/srcutiny.h
>>>> @@ -15,7 +15,7 @@
>>>>    struct srcu_struct {
>>>>        short srcu_lock_nesting[2];    /* srcu_read_lock() nesting 
>>>> depth. */
>>>> -    short srcu_idx;            /* Current reader array element. */
>>>> +    unsigned short srcu_idx;    /* Current reader array element in 
>>>> bit 0x2. */
>>>>        u8 srcu_gp_running;        /* GP workqueue running? */
>>>>        u8 srcu_gp_waiting;        /* GP waiting for readers? */
>>>>        struct swait_queue_head srcu_wq;
>>>> @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct 
>>>> srcu_struct *ssp)
>>>>    {
>>>>        int idx;
>>>> -    idx = READ_ONCE(ssp->srcu_idx);
>>>> +    idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;
>>>>        WRITE_ONCE(ssp->srcu_lock_nesting[idx], 
>>>> ssp->srcu_lock_nesting[idx] + 1);
>>>>        return idx;
>>>>    }
>>>
>>> Need change in idx calcultion in srcu_torture_stats_print() ?
>>>
>>> static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
>>>    idx = READ_ONCE(ssp->srcu_idx) & 0x1;
>>
>> Excellent point!  It should match the calculation in __srcu_read_lock(),
>> shouldn't it?  I have updated this, thank you!
>>
>>                             Thanx, Paul
>>
> 
> Updated version looks good!
> 
> 
> Thanks
> Neeraj
> 

For the version in rcu -dev:

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

Only minor point which I have is, the idx calculation can be made an 
inline func (though srcu_drive_gp() does not require a READ_ONCE for 
->srcu_idx):

__srcu_read_lock() and srcu_torture_stats_print() are using

idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1;

whereas srcu_drive_gp() uses:

idx = (ssp->srcu_idx & 0x2) / 2;

Thanks
Neeraj

>>> Thanks
>>> Neeraj
>>>
>>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>>>> index 6208c1d..5598cf6 100644
>>>> --- a/kernel/rcu/srcutiny.c
>>>> +++ b/kernel/rcu/srcutiny.c
>>>> @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp)
>>>>        ssp->srcu_cb_head = NULL;
>>>>        ssp->srcu_cb_tail = &ssp->srcu_cb_head;
>>>>        local_irq_enable();
>>>> -    idx = ssp->srcu_idx;
>>>> -    WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
>>>> +    idx = (ssp->srcu_idx & 0x2) / 2;
>>>> +    WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
>>>>        WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* 
>>>> srcu_read_unlock() wakes! */
>>>>        swait_event_exclusive(ssp->srcu_wq, 
>>>> !READ_ONCE(ssp->srcu_lock_nesting[idx]));
>>>>        WRITE_ONCE(ssp->srcu_gp_waiting, false); /* 
>>>> srcu_read_unlock() cheap. */
>>>> +    WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
>>>>        /* Invoke the callbacks we removed above. */
>>>>        while (lh) {
>>>>
>>>
>>> -- 
>>> 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