Exactly! At the same time you also don't want to call into the kernel's internal malloc() whenever a thread ends up blocking on a lock to allocate the data structures that are needed to keep track of queues of blocked threads for a given lock.
To prevent that, many operating systems allocate these 'queue objects' whenever threads are created and will attach a pointer to it from the thread object. Whenever a thread then stumbles upon a contended lock, it will effectively 'donate' this queue object to that lock, meaning that every lock having one or more waiters will have a linked list of 'queue objects' attached to it. When threads are woken up, they will each take one of those objects with them on the way out. But there's no guarantee that they will get their own queue object back; they may get shuffled! So by the time a thread terminates, it will free one of those objects, but that may not necessarily be the one it created.
I think the first operating system to use this method was Solaris. There they called these 'queue objects' turnstiles. The BSDs adopted the same approach, and kept the same name.
To prevent that, many operating systems allocate these 'queue objects' whenever threads are created and will attach a pointer to it from the thread object. Whenever a thread then stumbles upon a contended lock, it will effectively 'donate' this queue object to that lock, meaning that every lock having one or more waiters will have a linked list of 'queue objects' attached to it. When threads are woken up, they will each take one of those objects with them on the way out. But there's no guarantee that they will get their own queue object back; they may get shuffled! So by the time a thread terminates, it will free one of those objects, but that may not necessarily be the one it created.
I think the first operating system to use this method was Solaris. There they called these 'queue objects' turnstiles. The BSDs adopted the same approach, and kept the same name.
https://www.oreilly.com/library/view/solaristm-internals-cor...
https://www.bsdcan.org/2012/schedule/attachments/195_locking...