Implement thread destruction

This change makes libpthread release almost every resource allocated for
a thread, including the kernel thread, its send right, its reply port
and its stack. This improves resource usage after peaks of activity
during which servers can create hundreds or even thousands of threads.

To achieve this, the library relies on the recently added
thread_terminate_release one-way GNU Mach RPC, which allows threads to
release their last resources along with terminating in a single
operation. The pthread_exit function unconditionally releases all the
resources it can, including other kernel objects (namely the port used
for blocking and waking up) and signal states. When releasing the
pthread structure, a reference counter is used so that joinable threads
remain available. Once the reference counter drops to 0, the pthread
structure can be recycled. Thread local storage (TLS) is also recycled
since it needs to remain allocated while terminating the thread, as it
is there that the reply port is stored. TLS could be released too, after
grabbing the reply port name, but it is difficult to make sure no RPC
involving a reply port is used afterwards, so the simpler solution of
recycling TLS was chosen.

* Makefile (libpthread-routines): Replace pt-thread-halt with
pt-thread-terminate.
* pthread/pt-alloc.c (initialize_pthread): Set reference counter and
unconditionally initialize new threads completely.
(__pthread_alloc): Remove call to __pthread_thread_halt, update calls
to initialize_pthread.
* pthread/pt-create.c (__pthread_create_internal): Don't attempt to
reuse stacks, handle reference counter, update failure handling.
* pthread/pt-dealloc.c: Include bits/pt-atomic.h.
(__pthread_dealloc): Make pthread structure available for reuse when
reference counter reaches 0.
* pthread/pt-detach.c (pthread_detach): Assume the target thread takes
care of its own resources and, as a result, simply unreference its
pthread struct.
* pthread/pt-exit.c (__pthread_exit): Release resources and terminate.
* pthread/pt-internal.h (struct __pthread): New `nr_refs' member.
(__pthread_alloc): Update description.
(__pthread_dealloc): Likewise.
(__pthread_thread_dealloc): Likewise.
(__pthread_thread_terminate): New declaration.
* pthread/pt-join.c (pthread_join): Assume the target thread takes care
of its own resources and, as a result, simply unreference its pthread
struct.
* sysdeps/mach/hurd/pt-sigstate-destroy.c
(__pthread_sigstate_destroy): Call _hurd_sigstate_delete.
* sysdeps/mach/hurd/pt-sigstate-init.c (__pthread_sigstate_init): Call
_hurd_thread_sigstate and _hurd_sigstate_set_global_rcv when
appropriate.
* sysdeps/mach/hurd/pt-sysdep.c (__pthread_create_internal): Prevent
the library from releasing the stack of the main thread.
* sysdeps/mach/hurd/pt-sysdep.h (PTHREAD_SYSDEP_MEMBERS): Remove
`have_kernel_resources' from the list of sysdep members.
* sysdeps/mach/pt-thread-alloc.c (__pthread_thread_alloc): Update
thread allocation.
* sysdeps/mach/pt-thread-dealloc.c (__pthread_thread_dealloc): Update
description.
* sysdeps/mach/pt-thread-halt.c: Remove file.
* sysdeps/mach/pt-thread-start.c (__pthread_thread_start): Fix the
conditions under which a thread should actually be started.
* sysdeps/mach/pt-thread-terminate.c: New file.

1 files changed, 5 insertions, 0 deletions

diff --git a/pthread/pt-dealloc.c b/pthread/pt-dealloc.c
index 92fe1fd..e324800 100644
--- a/pthread/pt-dealloc.c
+++ b/pthread/pt-dealloc.c
@@ -23,6 +23,8 @@
 
 #include <pt-internal.h>
 
+#include <bits/pt-atomic.h>
+
 /* List of thread structures corresponding to free thread IDs.  */
 extern struct __pthread *__pthread_free_threads;
 extern pthread_mutex_t __pthread_free_threads_lock;
@@ -34,6 +36,9 @@ __pthread_dealloc (struct __pthread *pthread)
 {
   assert (pthread->state != PTHREAD_TERMINATED);
 
+  if (! __atomic_dec_and_test (&pthread->nr_refs))
+    return;
+
   /* Withdraw this thread from the thread ID lookup table.  */
   __pthread_setid (pthread->thread, NULL);