Re: [PATCH] rqspinlock: Fix order in raw_res_spin_(un)lock_irq to allow schedule

[Waiman Long <longman@redhat.com>]

On 6/9/26 7:22 AM, Arnd Bergmann wrote:
> On Tue, Jun 9, 2026, at 11:49, Gabriele Monaco wrote:
>> raw_res_spin_unlock_irqrestore() calls raw_res_spin_unlock() and then
>> restores interrupts, this means preemption is enabled when interrupts
>> are still disabled (as part of raw_res_spin_unlock()) so this cannot
>> trigger an actual preemption.
>> This is inconsistent with other spinlock implementations
>> (raw_spin_unlock_irqrestore() and bpf_res_spin_unlock_irqrestore()
>> itself).
>>
>> Adjust the macro to ensure interrupts are enabled before enabling
>> preemption, allowing to schedule at that point. Make the same
>> modification in the error path of raw_res_spin_lock_irqsave().
>>
>> Fixes: 101acd2e78b1 ("rqspinlock: Add macros for rqspinlock usage")
> Should this be Cc:stable@vger.kernel.org to get backported?
>
> Did you see this cause measurable performance problems,
> or did you find it through inspection?
>
>> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de> # asm-generic
>
> This should probably get merged through the BPF tree, but I've
> added the kernel/locking maintainers to Cc as well, since I
> feel it's more useful to have them look at it than me.
>
> Maybe it would be good to update (as a separate patch) the
> MAINTAINERS file so the locking subsystem also includes the
> headers currently missing:
>
> arch/*/include/asm/*spinlock*.h
> arch/*/include/asm/*rwlock*.h
> include/asm-generic/*spinlock*.h
> include/asm-generic/*rwlock*.h
>
>         Arnd
>
> (full patch quoted below)
>
>> ---
>>   include/asm-generic/rqspinlock.h | 14 +++++++++++---
>>   1 file changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/asm-generic/rqspinlock.h
>> b/include/asm-generic/rqspinlock.h
>> index 151d267a49..4d46643f46 100644
>> --- a/include/asm-generic/rqspinlock.h
>> +++ b/include/asm-generic/rqspinlock.h
>> @@ -243,12 +243,20 @@ static __always_inline void
>> res_spin_unlock(rqspinlock_t *lock)
>>   	({                                        \
>>   		int __ret;                        \
>>   		local_irq_save(flags);            \
>> -		__ret = raw_res_spin_lock(lock);  \
>> -		if (__ret)                        \
>> +		preempt_disable();                \
>> +		__ret = res_spin_lock(lock);      \
>> +		if (__ret) {                      \
>>   			local_irq_restore(flags); \
>> +			preempt_enable();         \
>> +		}                                 \
>>   		__ret;                            \
>>   	})
>>
>> -#define raw_res_spin_unlock_irqrestore(lock, flags) ({
>> raw_res_spin_unlock(lock); local_irq_restore(flags); })
>> +#define raw_res_spin_unlock_irqrestore(lock, flags) \
>> +	({                                          \
>> +		res_spin_unlock(lock);              \
>> +		local_irq_restore(flags);           \
>> +		preempt_enable();                   \
>> +	})
>>
>>   #endif /* __ASM_GENERIC_RQSPINLOCK_H */
>>
>> base-commit: e43ffb69e0438cddd72aaa30898b4dc446f664f8
>> -- 
>> 2.54.0

Disabling interrupt will also disable preemption. However, the possible 
side effect of scheduler preemption when preemption is enabled will make 
these res_spin_lock APIs behave more like regular spinlock. So

Acked-by: Waiman Long <longman@redhat.com>