Jeremie Koenig 653d74f12a hurd: Global signal disposition ... This adds _hurd_sigstate_set_global_rcv used by libpthread to enable POSIX-confirming behavior of signals on a per-thread basis. This also provides a sigstate destructor _hurd_sigstate_delete, and a global process signal state, which needs to be locked and check when global disposition is enabled, thus the addition of _hurd_sigstate_lock _hurd_sigstate_actions _hurd_sigstate_pending _hurd_sigstate_unlock helpers. This also updates all the glibc code accordingly. This also drops support for get_int(INIT_SIGMASK), which did not make sense any more since we do not have a single signal thread any more. During fork/spawn, this also reinitializes the child global sigstate's lock. That cures an issue that would very rarely cause a deadlock in the child in fork, tries to unlock ss' critical section lock at the end of fork. This will typically (always?) be observed in /bin/sh, which is not surprising as that is the foremost caller of fork. To reproduce an intermediate state, add an endless loop if _hurd_global_sigstate is locked after __proc_dostop (cast through volatile); that is, while still being in the fork's parent process. When that triggers (use the libtool testsuite), the signal thread has already locked ss (which is _hurd_global_sigstate), and is stuck at hurdsig.c:685 in post_signal, trying to lock _hurd_siglock (which the main thread already has locked and keeps locked until after __task_create). This is the case that ss->thread == MACH_PORT_NULL, that is, a global signal. In the main thread, between __proc_dostop and __task_create is the __thread_abort call on the signal thread which would abort any current kernel operation (but leave ss locked). Later in fork, in the parent, when _hurd_siglock is unlocked in fork, the parent's signal thread can proceed and will unlock eventually the global sigstate. In the client, _hurd_siglock will likewise be unlocked, but the global sigstate never will be, as the client's signal thread has been configured to restart execution from _hurd_msgport_receive. Thus, when the child tries to unlock ss' critical section lock at the end of fork, it will first lock the global sigstate, will spin trying to lock it, which can never be successful, and we get our deadlock. Options seem to be: * Move the locking of _hurd_siglock earlier in post_signal -- but that may generally impact performance, if this locking isn't generally needed anyway? On the other hand, would it actually make sense to wait here until we are not any longer in a critical section (which is meant to disable signal delivery anyway (but not for preempted signals?))? * Clear the global sigstate in the fork's child with the rationale that we're anyway restarting the signal thread from a clean state. This has now been implemented. Why has this problem not been observed before Jérémie's patches? (Or has it? Perhaps even more rarely?) In _S_msg_sig_post, the signal is now posted to a *global receiver thread*, whereas previously it was posted to the *designated signal-receiving thread*. The latter one was in a critical section in fork, so didn't try to handle the signal until after leaving the critical section? (Not completely analyzed and verified.) Another question is what the signal is that is being received during/around the time __proc_dostop executes.		2019-12-29 18:32:49 +01:00
..
hurd	hurd: Global signal disposition	2019-12-29 18:32:49 +01:00
hurd.h	hurd: Fix hurd installed headers test	2018-04-20 00:16:40 +02:00