threads: Update PID limit comment according to futex UAPI change

  address of the associated 'lock entry', plus or minus, of what will

  be called the 'lock word', from that 'lock entry'.  The 'lock word'

  is always a 32 bit word, unlike the other words above.  The 'lock

  word' holds 3 flag bits in the upper 3 bits, and the thread id (TID)

  of the thread holding the lock in the bottom 29 bits.  See further

  word' holds 2 flag bits in the upper 2 bits, and the thread id (TID)

  of the thread holding the lock in the bottom 30 bits.  See further

  below for a description of the flag bits.

  The third word, called 'list_op_pending', contains transient copy of

A given futex lock structure in a user shared memory region may be held

at different times by any of the threads with access to that region. The

thread currently holding such a lock, if any, is marked with the threads

TID in the lower 29 bits of the 'lock word'.

TID in the lower 30 bits of the 'lock word'.

When adding or removing a lock from its list of held locks, in order for

the kernel to correctly handle lock cleanup regardless of when the task

 1) set the 'list_op_pending' word to the address of the 'lock entry'

    to be inserted,

 2) acquire the futex lock,

 3) add the lock entry, with its thread id (TID) in the bottom 29 bits

 3) add the lock entry, with its thread id (TID) in the bottom 30 bits

    of the 'lock word', to the linked list starting at 'head', and

 4) clear the 'list_op_pending' word.

On exit, the kernel will consider the address stored in

'list_op_pending' and the address of each 'lock word' found by walking

the list starting at 'head'.  For each such address, if the bottom 29

the list starting at 'head'.  For each such address, if the bottom 30

bits of the 'lock word' at offset 'offset' from that address equals the

exiting threads TID, then the kernel will do two things:

    future kernel configuration changes) elements.

When the kernel sees a list entry whose 'lock word' doesn't have the

current threads TID in the lower 29 bits, it does nothing with that

current threads TID in the lower 30 bits, it does nothing with that

entry, and goes on to the next entry.

Bit 29 (0x20000000) of the 'lock word' is reserved for future use.

/*

 * 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))