linux-kernel - Re: [PATCH] atomic: Specify natural alignment for atomic

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Message-ID: <96ae7afc-c882-4c3d-9dea-3e2ae2789caf@linux.dev>
Date: Mon, 25 Aug 2025 13:00:06 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: akpm@...ux-foundation.org, geert@...ux-m68k.org,
 linux-kernel@...r.kernel.org, mhiramat@...nel.org, oak@...sinkinet.fi,
 peterz@...radead.org, stable@...r.kernel.org, will@...nel.org,
 Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t



On 2025/8/25 12:07, Finn Thain wrote:
> 
> On Mon, 25 Aug 2025, Lance Yang wrote:
> 
>>
>> Perhaps we should also apply the follwoing?
>>
>> diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
>> index 34e615c76ca5..940f8f3558f6 100644
>> --- a/include/linux/hung_task.h
>> +++ b/include/linux/hung_task.h
>> @@ -45,7 +45,7 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
>>   	 * If the lock pointer matches the BLOCKER_TYPE_MASK, return
>>   	 * without writing anything.
>>   	 */
>> -	if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK))
>> +	if (lock_ptr & BLOCKER_TYPE_MASK)
>>   		return;
>>
>>   	WRITE_ONCE(current->blocker, lock_ptr | type);
>> @@ -53,8 +53,6 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
>>
>>   static inline void hung_task_clear_blocker(void)
>>   {
>> -	WARN_ON_ONCE(!READ_ONCE(current->blocker));
>> -
>>   	WRITE_ONCE(current->blocker, 0UL);
>>   }
>>
>> Let the feature gracefully do nothing on that ;)
>>
> 
> This is poor style indeed.

Thanks for the lesson!

> 
> The conditional you've added to the hung task code has no real relevance
> to hung tasks. It doesn't belong there.

You're right! The original pointer-encoding was a deliberate trade-off to
save a field in task_struct, but as we're seeing now, that assumption is
fragile and causing issues :(

> 
> Of course, nobody wants that sort of logic to get duplicated at each site
> affected by the architectural quirk in question. Try to imagine if the
> whole kernel followed your example, and such unrelated conditionals were
> scattered across code base for a few decades. Now imagine trying to work
> on that code.

I agree with you completely: scattering more alignment checks into core 
logic
isn't the right long-term solution. It's not a clean design :(

> 
> You can see special cases for architectural quirks in drivers, but we do
> try to avoid them. And this is not a driver.

So, how about this?

What if we squash the runtime check fix into your patch? That would create a
single, complete fix that can be cleanly backported to stop all the spurious
warnings at once.

Then, as a follow-up, we can work on the proper long-term solution: changing
the pointer-encoding and re-introducing a dedicated field for the 
blocker type.