[PATCH] arm: spinlock: const qualify read-only functions.

[Ard Biesheuvel]

On 31 October 2015 at 18:26, Ian Coolidge <icoolidge at google.com> wrote:
> On Thu, Oct 29, 2015 at 5:27 PM, Ian Coolidge <icoolidge at google.com> wrote:
>> This allows assert_spin_locked() to be used against
>> spinlocks that are const qualified.
>>
>> For example:
>>
>> struct instance_t {
>>         int data;
>>         spinlock_t lock;
>> };
>>
>> /*
>>  * foo does not mutate instance.
>>  * foo must be called while the lock is held.
>>  */
>> int foo(const instance_t *instance) {
>>         assert_spin_locked(&instance->lock);
>>
>>         /* Code that assumes the lock is held */
>>
>>         return 0;
>> }
>>
>> Signed-off-by: Ian Coolidge <icoolidge at google.com>
>> ---
>>  arch/arm/include/asm/spinlock.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/spinlock.h b/arch/arm/include/asm/spinlock.h
>> index 0fa4184..274ceb1 100644
>> --- a/arch/arm/include/asm/spinlock.h
>> +++ b/arch/arm/include/asm/spinlock.h
>> @@ -118,12 +118,12 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
>>         return lock.tickets.owner == lock.tickets.next;
>>  }
>>
>> -static inline int arch_spin_is_locked(arch_spinlock_t *lock)
>> +static inline int arch_spin_is_locked(const arch_spinlock_t *lock)
>>  {
>>         return !arch_spin_value_unlocked(READ_ONCE(*lock));
>>  }
>>
>> -static inline int arch_spin_is_contended(arch_spinlock_t *lock)
>> +static inline int arch_spin_is_contended(const arch_spinlock_t *lock)
>>  {
>>         struct __raw_tickets tickets = READ_ONCE(lock->tickets);
>>         return (tickets.next - tickets.owner) > 1;
>> --
>> 2.6.0.rc2.230.g3dd15c0
>>
>
> Arnd,
>
> Please see this patch for an updated version. I should have noted it
> is PATCH v2 and replied to the old thread, sorry about that.
> Anyways, I provided an example of a const spinlock. Here, in that
> example, the spinlock is not const itself, but it is part of a
> structure that may be const.
> I believe the example is a good one.
>
> Also, with regards to your feedback, I don't think the length of the
> function body is a good argument against promising not to mutate it.
>

Hi,

Note that Ard (me) and Arnd are two different persons.

I think your patch makes sense in itself, I just wonder if there are
any real examples in the current codebase that would require it. Which
struct is it that you are looking to constify, and in which context?

Regards,
Ard.