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[PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor

Paul E. McKenney posted 12 patches 1 month, 2 weeks ago
There is a newer version of this series
[PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
Posted by Paul E. McKenney 1 month, 2 weeks ago 
Currently, there are only two flavors of readers, normal and NMI-safe.
Very straightforward state updates suffice to check for erroneous
mixing of reader flavors on a given srcu_struct structure.  This commit
upgrades the checking in preparation for the addition of light-weight
(as in memory-barrier-free) readers.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
---
 kernel/rcu/srcutree.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 18f2eae5e14bd..abe55777c4335 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
 		if (IS_ENABLED(CONFIG_PROVE_RCU))
 			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
 	}
-	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
+	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
 		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
 	return sum;
 }
@@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 	sdp = raw_cpu_ptr(ssp->sda);
 	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
 	if (!old_reader_flavor_mask) {
-		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
-		return;
+		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
+		if (!old_reader_flavor_mask)
+			return;
 	}
 	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
 }
-- 
2.40.1
Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
Posted by Neeraj Upadhyay 1 month, 2 weeks ago 
On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
> Currently, there are only two flavors of readers, normal and NMI-safe.
> Very straightforward state updates suffice to check for erroneous
> mixing of reader flavors on a given srcu_struct structure.  This commit
> upgrades the checking in preparation for the addition of light-weight
> (as in memory-barrier-free) readers.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: <bpf@vger.kernel.org>
> ---
>  kernel/rcu/srcutree.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 18f2eae5e14bd..abe55777c4335 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>  		if (IS_ENABLED(CONFIG_PROVE_RCU))
>  			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
>  	}
> -	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
> +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
>  		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
>  	return sum;
>  }
> @@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
>  	sdp = raw_cpu_ptr(ssp->sda);
>  	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
>  	if (!old_reader_flavor_mask) {
> -		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
> -		return;
> +		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);

This looks to be separate independent fix?


- Neeraj

> +		if (!old_reader_flavor_mask)
> +			return;
>  	}
>  	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
>  }
Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
Posted by Paul E. McKenney 1 month, 1 week ago 
On Mon, Oct 14, 2024 at 02:42:33PM +0530, Neeraj Upadhyay wrote:
> On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
> > Currently, there are only two flavors of readers, normal and NMI-safe.
> > Very straightforward state updates suffice to check for erroneous
> > mixing of reader flavors on a given srcu_struct structure.  This commit
> > upgrades the checking in preparation for the addition of light-weight
> > (as in memory-barrier-free) readers.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Andrii Nakryiko <andrii@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Kent Overstreet <kent.overstreet@linux.dev>
> > Cc: <bpf@vger.kernel.org>
> > ---
> >  kernel/rcu/srcutree.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 18f2eae5e14bd..abe55777c4335 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
> >  		if (IS_ENABLED(CONFIG_PROVE_RCU))
> >  			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
> >  	}
> > -	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
> > +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
> >  		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
> >  	return sum;
> >  }
> > @@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
> >  	sdp = raw_cpu_ptr(ssp->sda);
> >  	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
> >  	if (!old_reader_flavor_mask) {
> > -		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
> > -		return;
> > +		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
> 
> This looks to be separate independent fix?

I would say that it is part of the upgrade.  The old logic worked if there
are only two flavors, but the cmpxchg() is required for more than two.

							Thanx, Paul

> - Neeraj
> 
> > +		if (!old_reader_flavor_mask)
> > +			return;
> >  	}
> >  	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
> >  }
>
Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
Posted by Neeraj Upadhyay 1 month, 1 week ago 

On 10/14/2024 10:21 PM, Paul E. McKenney wrote:
> On Mon, Oct 14, 2024 at 02:42:33PM +0530, Neeraj Upadhyay wrote:
>> On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
>>> Currently, there are only two flavors of readers, normal and NMI-safe.
>>> Very straightforward state updates suffice to check for erroneous
>>> mixing of reader flavors on a given srcu_struct structure.  This commit
>>> upgrades the checking in preparation for the addition of light-weight
>>> (as in memory-barrier-free) readers.
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>> Cc: Alexei Starovoitov <ast@kernel.org>
>>> Cc: Andrii Nakryiko <andrii@kernel.org>
>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>> Cc: Kent Overstreet <kent.overstreet@linux.dev>
>>> Cc: <bpf@vger.kernel.org>
>>> ---
>>>  kernel/rcu/srcutree.c | 7 ++++---
>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>> index 18f2eae5e14bd..abe55777c4335 100644
>>> --- a/kernel/rcu/srcutree.c
>>> +++ b/kernel/rcu/srcutree.c
>>> @@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>>>  		if (IS_ENABLED(CONFIG_PROVE_RCU))
>>>  			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
>>>  	}
>>> -	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
>>> +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
>>>  		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
>>>  	return sum;
>>>  }
>>> @@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
>>>  	sdp = raw_cpu_ptr(ssp->sda);
>>>  	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
>>>  	if (!old_reader_flavor_mask) {
>>> -		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
>>> -		return;
>>> +		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
>>
>> This looks to be separate independent fix?
> 
> I would say that it is part of the upgrade.  The old logic worked if there
> are only two flavors, but the cmpxchg() is required for more than two.
> 

Ok, I need to check more to understand why it is not required when we
have only two flavors.


- Neeraj

> 							Thanx, Paul
> 
>> - Neeraj
>>
>>> +		if (!old_reader_flavor_mask)
>>> +			return;
>>>  	}
>>>  	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
>>>  }
>>

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