From e07fbce216ec8da5e1a6583019d0292dab28f6d2 Mon Sep 17 00:00:00 2001 From: Taylor R Campbell Date: Sun, 16 Jul 2023 22:14:02 +0000 Subject: [PATCH] kthread(9): Fix nested kthread_join. No reason for one kthread_join to interfere with another, or to cause non-cyclic dependencies to get stuck. XXX Currently abuses struct lwp::l_private for two flags, KJ_JOINED and KJ_EXITED. I don't think anything in kthread(9) uses l_private, but maybe this should use l__reserved or something instead, or maybe we should create a union member for the purpose. --- sys/kern/kern_kthread.c | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/sys/kern/kern_kthread.c b/sys/kern/kern_kthread.c index eddfc7185cf9..d2fbb32227f5 100644 --- a/sys/kern/kern_kthread.c +++ b/sys/kern/kern_kthread.c @@ -45,7 +45,15 @@ __KERNEL_RCSID(0, "$NetBSD: kern_kthread.c,v 1.47 2022/09/13 09:37:49 riastradh #include -static lwp_t * kthread_jtarget; +/* XXX */ +#define kthread_joinflags(l) \ + ((uintptr_t)(l)->l_private) +#define kthread_setjoinflags(l, f) \ + ((l)->l_private = (void *)((uintptr_t)(l)->l_private | (uintptr_t)(f))) + +#define KJ_JOINING __BIT(0) +#define KJ_EXITED __BIT(1) + static kmutex_t kthread_lock; static kcondvar_t kthread_cv; @@ -55,7 +63,6 @@ kthread_sysinit(void) mutex_init(&kthread_lock, MUTEX_DEFAULT, IPL_NONE); cv_init(&kthread_cv, "kthrwait"); - kthread_jtarget = NULL; } /* @@ -173,10 +180,11 @@ kthread_exit(int ecode) /* Barrier for joining. */ if (l->l_pflag & LP_MUSTJOIN) { mutex_enter(&kthread_lock); - while (kthread_jtarget != l) { + KASSERT((kthread_joinflags(l) & KJ_EXITED) == 0); + while ((kthread_joinflags(l) & KJ_JOINING) == 0) { cv_wait(&kthread_cv, &kthread_lock); } - kthread_jtarget = NULL; + kthread_setjoinflags(l, KJ_EXITED); cv_broadcast(&kthread_cv); mutex_exit(&kthread_lock); } @@ -202,17 +210,14 @@ kthread_join(lwp_t *l) KASSERT((l->l_pflag & LP_MUSTJOIN) != 0); /* - * - Wait if some other thread has occupied the target. * - Specify our kthread as a target and notify it. * - Wait for the target kthread to notify us. */ mutex_enter(&kthread_lock); - while (kthread_jtarget) { - cv_wait(&kthread_cv, &kthread_lock); - } - kthread_jtarget = l; + KASSERT((kthread_joinflags(l) & KJ_JOINING) == 0); + kthread_setjoinflags(l, KJ_JOINING); cv_broadcast(&kthread_cv); - while (kthread_jtarget == l) { + while ((kthread_joinflags(l) & KJ_EXITED) == 0) { cv_wait(&kthread_cv, &kthread_lock); } mutex_exit(&kthread_lock);