Re: [PATCH] threads: Update PID limit comment according to futex UAPI change

[Thomas Gleixner]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> On Mon, Mar 02, 2020 at 12:29:39PM +0100, Jann Horn wrote:
>> The futex UAPI changed back in commit 76b81e2b0e22 ("[PATCH] lightweight
>> robust futexes updates 2"), which landed in v2.6.17: FUTEX_TID_MASK is now
>> 0x3fffffff instead of 0x1fffffff. Update the corresponding comment in
>> include/linux/threads.h.
>> 
>> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
>> ---
>>  include/linux/threads.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/include/linux/threads.h b/include/linux/threads.h
>> index 3086dba525e20..18d5a74bcc3dd 100644
>> --- a/include/linux/threads.h
>> +++ b/include/linux/threads.h
>> @@ -29,7 +29,7 @@
>>  
>>  /*
>>   * A maximum of 4 million PIDs should be enough for a while.
>> - * [NOTE: PID/TIDs are limited to 2^29 ~= 500+ million, see futex.h.]
>> + * [NOTE: PID/TIDs are limited to 2^30 ~= 1 billion, see FUTEX_TID_MASK.]
>>   */
>>  #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
>>  	(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))
>
> I just noticed another mention of this in Documentation/robust-futex-ABI.txt
> There it states that bit-29 is reserved for future use.
>
> Thomas, do we want to release that bit and update all this?

In fact we've released it long ago:

include/uapi/linux/futex.h:

#define FUTEX_TID_MASK          0x3fffffff

Thanks,

        tglx