linux-kernel - Re: [PATCH] locking/pvqspinlock: Correct the type of "old" variable in pv_kick

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Message-ID: <64b0aba3-6eab-4418-9019-a7e1123fba96@redhat.com>
Date: Mon, 22 Jul 2024 11:13:02 -0400
From: Waiman Long <longman@...hat.com>
To: maobibo <maobibo@...ngson.cn>, Uros Bizjak <ubizjak@...il.com>,
 linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] locking/pvqspinlock: Correct the type of "old" variable
 in pv_kick_node()


On 7/22/24 02:54, maobibo wrote:
> Uros,
>
> Sorry for late reply because of weekend time. This modification works 
> well for me.
>
> In later I want to define pv_node::state as int type on LoongArch 
> because there is only int32/int64 cmpxchg is better supported on the 
> system, however that is another issue.

I thought about changing pv_node::state to int, but that requires 
changing the pv_wait() call to use int too. That doesn't work because 
pv_wait() is also used to monitor the state of the lock byte of the 
qspinlock. In essence, we can't use pvqspinlock if 8-bit cmpxchg isn't 
supported.

Cheers,
Longman

>
> Tested-by:Bibo Mao <maobibo@...ngson.cn>
>
> On 2024/7/22 上午12:45, Uros Bizjak wrote:
>> "enum vcpu_state" is not compatible with "u8" type for all targets,
>> resulting in:
>>
>> error: initialization of 'u8 *' {aka 'unsigned char *'} from 
>> incompatible pointer type 'enum vcpu_state *'
>>
>> for LoongArch. Correct the type of "old" variable to "u8".
>>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Waiman Long <longman@...hat.com>
>> Cc: Boqun Feng <boqun.feng@...il.com>
>> Fixes: fea0e1820b51 ("locking/pvqspinlock: Use try_cmpxchg() in 
>> qspinlock_paravirt.h")
>> Reported-by: Bibo Mao <maobibo@...ngson.cn>
>> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>> Closes: 
>> https://lore.kernel.org/lkml/20240719024010.3296488-1-maobibo@loongson.cn/
>> ---
>>   kernel/locking/qspinlock_paravirt.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/locking/qspinlock_paravirt.h 
>> b/kernel/locking/qspinlock_paravirt.h
>> index f5a36e67b593..ac2e22502741 100644
>> --- a/kernel/locking/qspinlock_paravirt.h
>> +++ b/kernel/locking/qspinlock_paravirt.h
>> @@ -357,7 +357,7 @@ static void pv_wait_node(struct mcs_spinlock 
>> *node, struct mcs_spinlock *prev)
>>   static void pv_kick_node(struct qspinlock *lock, struct 
>> mcs_spinlock *node)
>>   {
>>       struct pv_node *pn = (struct pv_node *)node;
>> -    enum vcpu_state old = vcpu_halted;
>> +    u8 old = vcpu_halted;
>>       /*
>>        * If the vCPU is indeed halted, advance its state to match 
>> that of
>>        * pv_wait_node(). If OTOH this fails, the vCPU was running and 
>> will
>>
>