Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
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
>>> Cc: Alexei Starovoitov
>>> Cc: Andrii Nakryiko
>>> Cc: Peter Zijlstra
>>> Cc: Kent Overstreet
>>> Cc:
>>> ---
>>> 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);
>>> }
>>
Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor
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
> > Cc: Alexei Starovoitov
> > Cc: Andrii Nakryiko
> > Cc: Peter Zijlstra
> > Cc: Kent Overstreet
> > Cc:
> > ---
> > 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
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
> Cc: Alexei Starovoitov
> Cc: Andrii Nakryiko
> Cc: Peter Zijlstra
> Cc: Kent Overstreet
> Cc:
> ---
> 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);
> }
