This is a private function that locks the lock much like the regular
pthread_mutex_lock(), but causes the corresponding unlock operation to
always assume there may be other waiters. This is useful in case some
waiters are made to wait on the mutex's futex directly, without going
through pthread_mutex_lock(). This is going to be used by the condition
variable implementation in the next commit.
pthread_mutex is now an actual "sleeping" mutex, and not just a
spinlock! It still has a fast path that only uses atomics and (in the
successful case) returns immediately without sleeping. In case of
contention, it calls futex_wait(), which lets the kernel scheduler put
this thread to sleep, *and* lets it know exactly when to consider
scheduling it again.
This ensures the store to mutex->lock doesn't get re-ordered before
the store to mutex->owner which could otherwise result in a locked
owner-less mutex if another thread tries to acquire the lock at
the same time.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
GCC will insert various calls to pthread functions when compiling
C++ code with static initializers, even when the user doesn't link
their program against libpthread explicitly.
This is used to make static initializers thread-safe, e.g. when
building a library that does not itself use thread functionality
and thus does not link against libpthread - but is intended to
be used with other code that does use libpthread explicitly.
This makes these symbols available in libc.
This required a bit of rearchitecture, as pthread_atfork() required a
mutex, and duplicating a mutex impl for it was silly.
As such, this patch moves some standalone bits of pthread into LibC and
uses those to implement atfork().
It should be noted that for programs that don't use atfork(), this
mechanism only costs two atomic loads (as opposed to the normal mutex
lock+unlock) :^)
