From: Neeraj Upadhyay <neeraju@codeaurora.org>
To: paulmck@kernel.org
Cc: 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,
rcu@vger.kernel.org
Subject: Re: [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter
Date: Thu, 19 Nov 2020 13:44:49 +0530 [thread overview]
Message-ID: <72dffe43-b746-6d75-1f6a-9936d709be63@codeaurora.org> (raw)
In-Reply-To: <20201117004052.14758-1-paulmck@kernel.org>
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. 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>
> 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..fed4a2d 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) & 0x2) / 2;
Should we use bit 0x2 of (READ_ONCE(ssp->srcu_idx) + 1) , if GP
(srcu_drive_gp()) is in progress? Or am I missing something here?
idx = ((READ_ONCE(ssp->srcu_idx) +1) & 0x2) / 2;
Also, any reason for using divison instead of shift; something to
do with 16-bit srcu_idx which I am missing here?
Thanks
Neeraj
> WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
> return idx;
> }
> 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
next prev parent reply other threads:[~2020-11-19 8:15 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-17 0:40 [PATCH tip/core/rcu 0/5] Provide SRCU polling grace-period interfaces Paul E. McKenney
2020-11-17 0:40 ` [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter paulmck
2020-11-19 8:14 ` Neeraj Upadhyay [this message]
2020-11-19 18:00 ` Paul E. McKenney
2020-11-17 0:40 ` [PATCH RFC tip/core/rcu 2/5] srcu: Provide internal interface to start a Tiny SRCU grace period paulmck
2020-11-20 11:36 ` Neeraj Upadhyay
2020-11-17 0:40 ` [PATCH RFC tip/core/rcu 3/5] srcu: Provide internal interface to start a Tree " paulmck
2020-11-20 11:36 ` Neeraj Upadhyay
2020-11-21 0:37 ` Paul E. McKenney
2020-11-17 0:40 ` [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods paulmck
2020-11-20 11:58 ` Neeraj Upadhyay
2020-11-21 0:13 ` Paul E. McKenney
2020-11-22 14:27 ` Neeraj Upadhyay
2020-11-22 18:01 ` Paul E. McKenney
2020-11-23 4:34 ` Neeraj Upadhyay
2020-11-23 21:07 ` Paul E. McKenney
2020-11-17 0:40 ` [PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces for Tree " paulmck
2020-11-20 12:01 ` Neeraj Upadhyay
2020-11-21 0:16 ` Paul E. McKenney
2020-11-22 14:22 ` Neeraj Upadhyay
2020-11-21 0:58 ` [PATCH tip/core/rcu 0/5] Provide SRCU polling grace-period interfaces Paul E. McKenney
2020-11-21 1:05 ` Steven Rostedt
2020-11-21 1:12 ` Paul E. McKenney
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