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Message-ID: <28a22477-9fbd-603a-404c-1cf6e7cc18b5@codeaurora.org>
Date: Wed, 25 Nov 2020 10:03:26 +0530
From: Neeraj Upadhyay <neeraju@...eaurora.org>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
kent.overstreet@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit
grace-period counter
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@...nel.org wrote:
>>>> From: "Paul E. McKenney" <paulmck@...nel.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@...il.com>
>>>> [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per
>>>> Upadhyay. ]
>>>> Signed-off-by: Paul E. McKenney <paulmck@...nel.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@...eaurora.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
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