linux-kernel - Re: [PATCH v4 1/7] sched/fair: Provide u64 read for 32-bits arch helper

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <5033835b-195f-2712-d621-c3c06cc61a08@arm.com>
Date:   Fri, 22 Apr 2022 12:10:36 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com, qperret@...gle.com
Subject: Re: [PATCH v4 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper

[...]

>> +#ifdef CONFIG_64BIT
>> +# define u64_u32_load_copy(var, copy)       var
>> +# define u64_u32_store_copy(var, copy, val) (var = val)
>> +#else
>> +# define u64_u32_load_copy(var, copy)					\
>> +({									\
>> +	u64 __val, __val_copy;						\
>> +	do {								\
>> +		__val_copy = copy;					\
>> +		/*							\
>> +		 * paired with u64_u32_store, ordering access		\
>> +		 * to var and copy.					\
>> +		 */							\
>> +		smp_rmb();						\
>> +		__val = var;						\
>> +	} while (__val != __val_copy);					\
>> +	__val;								\
>> +})
>> +# define u64_u32_store_copy(var, copy, val)				\
>> +do {									\
>> +	typeof(val) __val = (val);					\
>> +	var = __val;							\
>> +	/*								\
>> +	 * paired with u64_u32_load, ordering access to var and		\
>> +	 * copy.							\
>> +	 */								\
>> +	smp_wmb();							\
>> +	copy = __val;							\
> 
> `copy = __val;` should be `copy = var`.
> 
> If var equal to val we do not need to do store. Check this condition
> in the above macro to avoid a redundant store.
> 
>   if (var != __val)
>     var = __val;

Judging by the users of this macro, var = val is very much unlikely to
happen. Also, I don't think we want to waste a if here.

[...]