Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit grace-period counter

Neeraj Upadhyay
2020-11-24T05:19:11Z

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

>> 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