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Date: Sat, 1 Oct 2022 09:51:52 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: John Ogness <john.ogness@...utronix.de>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, rostedt@...dmis.org,
Thomas Gleixner <tglx@...utronix.de>,
Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH RFC v2 rcu 1/8] srcu: Convert ->srcu_lock_count and
->srcu_unlock_count to atomic
On Fri, Sep 30, 2022 at 10:43:43PM +0206, John Ogness wrote:
> On 2022-09-30, "Paul E. McKenney" <paulmck@...nel.org> wrote:
> >> > - this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
> >> > + this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
> >>
> >> Is there any particular reason that you are directly modifying
> >> @counter instead of raw_cpu_ptr()+atomic_long_inc() that do you in
> >> __srcu_read_lock_nmisafe() of patch 2?
> >
> > Performance. From what I can see, this_cpu_inc() is way faster than
> > atomic_long_inc() on x86 and s390. Maybe also on loongarch. No idea
> > on arm64.
>
> Yeah, that's what I figured. I just wanted to make sure.
>
> FWIW, the rest of the series looks pretty straight forward to me.
Thank you for looking it over! The updated patch is shown below.
The full series is in -rcu at branch srcunmisafe.2022.09.30a. Feel free
to pull it into your printk() series if that makes things easier for you.
Thanx, Paul
------------------------------------------------------------------------
commit 24511d0b754db760d4e1a08fc48a180f6a5a948b
Author: Paul E. McKenney <paulmck@...nel.org>
Date: Thu Sep 15 12:09:30 2022 -0700
srcu: Convert ->srcu_lock_count and ->srcu_unlock_count to atomic
NMI-safe variants of srcu_read_lock() and srcu_read_unlock() are needed
by printk(), which on many architectures entails read-modify-write
atomic operations. This commit prepares Tree SRCU for this change by
making both ->srcu_lock_count and ->srcu_unlock_count by atomic_long_t.
[ paulmck: Apply feedback from John Ogness. ]
Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Petr Mladek <pmladek@...e.com>
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index e3014319d1ad..0c4eca07d78d 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -23,8 +23,8 @@ struct srcu_struct;
*/
struct srcu_data {
/* Read-side state. */
- unsigned long srcu_lock_count[2]; /* Locks per CPU. */
- unsigned long srcu_unlock_count[2]; /* Unlocks per CPU. */
+ atomic_long_t srcu_lock_count[2]; /* Locks per CPU. */
+ atomic_long_t srcu_unlock_count[2]; /* Unlocks per CPU. */
/* Update-side state. */
spinlock_t __private lock ____cacheline_internodealigned_in_smp;
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 1c304fec89c0..25e9458da6a2 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -417,7 +417,7 @@ static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
- sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
+ sum += atomic_long_read(&cpuc->srcu_lock_count[idx]);
}
return sum;
}
@@ -434,7 +434,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
- sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
+ sum += atomic_long_read(&cpuc->srcu_unlock_count[idx]);
}
return sum;
}
@@ -503,10 +503,10 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
- sum += READ_ONCE(cpuc->srcu_lock_count[0]);
- sum += READ_ONCE(cpuc->srcu_lock_count[1]);
- sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
- sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
+ sum += atomic_long_read(&cpuc->srcu_lock_count[0]);
+ sum += atomic_long_read(&cpuc->srcu_lock_count[1]);
+ sum -= atomic_long_read(&cpuc->srcu_unlock_count[0]);
+ sum -= atomic_long_read(&cpuc->srcu_unlock_count[1]);
}
return sum;
}
@@ -636,7 +636,7 @@ int __srcu_read_lock(struct srcu_struct *ssp)
int idx;
idx = READ_ONCE(ssp->srcu_idx) & 0x1;
- this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
+ this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
smp_mb(); /* B */ /* Avoid leaking the critical section. */
return idx;
}
@@ -650,7 +650,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
{
smp_mb(); /* C */ /* Avoid leaking the critical section. */
- this_cpu_inc(ssp->sda->srcu_unlock_count[idx]);
+ this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter);
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);
@@ -1687,8 +1687,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
struct srcu_data *sdp;
sdp = per_cpu_ptr(ssp->sda, cpu);
- u0 = data_race(sdp->srcu_unlock_count[!idx]);
- u1 = data_race(sdp->srcu_unlock_count[idx]);
+ u0 = data_race(atomic_long_read(&sdp->srcu_unlock_count[!idx]));
+ u1 = data_race(atomic_long_read(&sdp->srcu_unlock_count[idx]));
/*
* Make sure that a lock is always counted if the corresponding
@@ -1696,8 +1696,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
*/
smp_rmb();
- l0 = data_race(sdp->srcu_lock_count[!idx]);
- l1 = data_race(sdp->srcu_lock_count[idx]);
+ l0 = data_race(atomic_long_read(&sdp->srcu_lock_count[!idx]));
+ l1 = data_race(atomic_long_read(&sdp->srcu_lock_count[idx]));
c0 = l0 - u0;
c1 = l1 - u1;
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