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c4c76d77cb
The existing g_pointer_bit_lock() and g_pointer_bit_unlock() API
requires the user to understand/reimplement how bits of the pointer get
mangled. Add helper functions for that.
The useful thing to do with g_pointer_bit_lock() API is to get/set
pointers while having it locked. For example, to set the pointer a user
can do:
g_pointer_bit_lock (&lockptr, lock_bit);
ptr2 = set_bit_pointer_as_if_locked(ptr, lock_bit);
g_atomic_pointer_set (&lockptr, ptr2);
g_pointer_bit_unlock (&lockptr, lock_bit);
That has several problems:
- it requires one extra atomic operations (3 instead of 2, in the
non-contended case).
- the first g_atomic_pointer_set() already wakes blocked threads,
which find themselves still being locked and needs to go back to
sleep.
- the user needs to re-implement how bit-locking mangles the pointer so
that it looks as if it were locked.
- while the user tries to re-implement what glib does to mangle the
pointer for bitlocking, there is no immediate guarantee that they get
it right.
Now we can do instead:
g_pointer_bit_lock(&lockptr, lock_bit);
g_pointer_bit_unlock_and_set(&lockptr, lock_bit, ptr, 0);
This will also emit a critical if @ptr has the locked bit set.
g_pointer_bit_lock() really only works with pointers that have a certain
alignment, and the lowest bits unset. Otherwise, there is no space to
encode both the locking and all pointer values. The new assertion helps
to catch such bugs.
Also, g_pointer_bit_lock_mask_ptr() is here, so we can do:
g_pointer_bit_lock(&lockptr, lock_bit);
/* set a pointer separately, when g_pointer_bit_unlock_and_set() is unsuitable. */
g_atomic_pointer_set(&lockptr, g_pointer_bit_lock_mask_ptr(ptr, lock_bit, TRUE, 0, NULL));
...
g_pointer_bit_unlock(&lockptr, lock_bit);
and:
g_pointer_bit_lock(&lockptr, lock_bit);
/* read the real pointer after getting the lock. */
ptr = g_pointer_bit_lock_mask_ptr(lockptr, lock_bit, FALSE, 0, NULL));
...
g_pointer_bit_unlock(&lockptr, lock_bit);