Le 06/04/2023 à 10:15, David Laight a écrit :
> From: Luis Chamberlain Luis Chamberlain
>> Sent: 05 April 2023 17:53
>>
>> On Wed, Apr 05, 2023 at 09:23:09AM -0700, Linus Torvalds wrote:
>>> On Wed, Apr 5, 2023 at 9:11 AM Luis Chamberlain <mcgrof@xxxxxxxxxx> wrote:
>>>>
>>>> Oh but I don't get the atomic incs, so we'd need debugfs_create_atomic_long_t().
>>>
>>> debugfs_create_ulong("total_mod_size",
>>> 0400, mod_debugfs_root,
>>> &total_mod_size.counter);
>>>
>>> but I didn't actually try to compile that kind of version.
>>>
>>> (I think "counter" is actually a _signed_ long, so maybe the types don't match).
>>
>> I see yes, sadly we'd have to cast to (unsigned long *) to make that
>> work as atomic_long counters are long long int:
>>
>> debugfs_create_ulong("total_mod_size",
>> 0400, mod_debugfs_root,
>> - &total_mod_size.counter);
>> + (unsigned long *)&total_mod_size.counter);
>>
>> That's:
>>
>> unsigned long min bits 32
>> long long min bits 64
>>
>> But since we'd be doing our accounting in atomic_long and just use
>> debugfs for prints I think that's fine. Just a bit ugly.
>
> That isn't going to work.
> It is pretty much never right to do *(integer_type *)&integer_variable.
>
> But are you really sure 'atomic_long' is 'long long'
> doesn't sound right at all.
> 'long long' is 128bit on 64bit and 64bit on 32bit - so is always
> a double-register access.
> This is worse than atomic_u64.
On powerpc 'long long' is 64 bits on both PPC32 and PPC64.
Christophe
>
> I should probably try to lookup and/or measure the performance
> of atomic operations (esp. cmpxchg) on x86.
> Historically they were a separate read and write bus cycles with
> the 'lock' signal asserted (and still are if they cross cache
> line boundaries).
> But it is possible that at least some of the locked operations
> (esp. the xchg ones) are implemented within the cache itself
> so are just a single cpu -> cache operation.
> If not it is actually possible that the _local variants that
> seem to have been added should not use the locked instructions!
>
> David
>
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