fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

mikechristie · torvalds · commit f9010dbdce91 · 2023-06-01T17:15:33.000-04:00
When switching from kthreads to vhost_tasks two bugs were added: 1. The vhost worker tasks's now show up as processes so scripts doing ps or ps a would not incorrectly detect the vhost task as another process. 2. kthreads disabled freeze by setting PF_NOFREEZE, but vhost tasks's didn't disable or add support for them. To fix both bugs, this switches the vhost task to be thread in the process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that SIGKILL/STOP support is required because CLONE_THREAD requires CLONE_SIGHAND which requires those 2 signals to be supported. This is a modified version of the patch written by Mike Christie <michael.christie@oracle.com> which was a modified version of patch originally written by Linus. Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER. Including ignoring signals, setting up the register state, and having get_signal return instead of calling do_group_exit. Tidied up the vhost_task abstraction so that the definition of vhost_task only needs to be visible inside of vhost_task.c. Making it easier to review the code and tell what needs to be done where. As part of this the main loop has been moved from vhost_worker into vhost_task_fn. vhost_worker now returns true if work was done. The main loop has been updated to call get_signal which handles SIGSTOP, freezing, and collects the message that tells the thread to exit as part of process exit. This collection clears __fatal_signal_pending. This collection is not guaranteed to clear signal_pending() so clear that explicitly so the schedule() sleeps. For now the vhost thread continues to exist and run work until the last file descriptor is closed and the release function is called as part of freeing struct file. To avoid hangs in the coredump rendezvous and when killing threads in a multi-threaded exec. The coredump code and de_thread have been modified to ignore vhost threads. Remvoing the special case for exec appears to require teaching vhost_dev_flush how to directly complete transactions in case the vhost thread is no longer running. Removing the special case for coredump rendezvous requires either the above fix needed for exec or moving the coredump rendezvous into get_signal. Fixes: 6e890c5 ("vhost: use vhost_tasks for worker threads") Signed-off-by: Eric W. Biederman <ebiederm@xmission.com> Co-developed-by: Mike Christie <michael.christie@oracle.com> Signed-off-by: Mike Christie <michael.christie@oracle.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
@@ -39,7 +39,7 @@ extern void fpu_flush_thread(void);
 static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
 	if (cpu_feature_enabled(X86_FEATURE_FPU) &&
-	    !(current->flags & (PF_KTHREAD | PF_IO_WORKER))) {
+	    !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
 		save_fpregs_to_fpstate(old_fpu);
 		/*
 		 * The save operation preserved register state, so the
diff --git a/arch/x86/kernel/fpu/context.h b/arch/x86/kernel/fpu/context.h
@@ -57,7 +57,7 @@ static inline void fpregs_restore_userregs(void)
 	struct fpu *fpu = &current->thread.fpu;
 	int cpu = smp_processor_id();
 
-	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_IO_WORKER)))
+	if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER)))
 		return;
 
 	if (!fpregs_state_valid(fpu, cpu)) {
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
@@ -426,7 +426,7 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
 
 	this_cpu_write(in_kernel_fpu, true);
 
-	if (!(current->flags & (PF_KTHREAD | PF_IO_WORKER)) &&
+	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
 	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
 		set_thread_flag(TIF_NEED_FPU_LOAD);
 		save_fpregs_to_fpstate(&current->thread.fpu);
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
@@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
 		 * test_and_set_bit() implies a memory barrier.
 		 */
 		llist_add(&work->node, &dev->worker->work_list);
-		wake_up_process(dev->worker->vtsk->task);
+		vhost_task_wake(dev->worker->vtsk);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);
@@ -333,39 +333,27 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	__vhost_vq_meta_reset(vq);
 }
 
-static int vhost_worker(void *data)
+static bool vhost_worker(void *data)
 {
 	struct vhost_worker *worker = data;
 	struct vhost_work *work, *work_next;
 	struct llist_node *node;
 
-	for (;;) {
-		/* mb paired w/ kthread_stop */
-		set_current_state(TASK_INTERRUPTIBLE);
-
-		if (vhost_task_should_stop(worker->vtsk)) {
-			__set_current_state(TASK_RUNNING);
-			break;
-		}
-
-		node = llist_del_all(&worker->work_list);
-		if (!node)
-			schedule();
-
+	node = llist_del_all(&worker->work_list);
+	if (node) {
 		node = llist_reverse_order(node);
 		/* make sure flag is seen after deletion */
 		smp_wmb();
 		llist_for_each_entry_safe(work, work_next, node, node) {
 			clear_bit(VHOST_WORK_QUEUED, &work->flags);
-			__set_current_state(TASK_RUNNING);
 			kcov_remote_start_common(worker->kcov_handle);
 			work->fn(work);
 			kcov_remote_stop();
 			cond_resched();
 		}
 	}
 
-	return 0;
+	return !!node;
 }
 
 static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
diff --git a/fs/coredump.c b/fs/coredump.c
@@ -371,7 +371,9 @@ static int zap_process(struct task_struct *start, int exit_code)
 		if (t != current && !(t->flags & PF_POSTCOREDUMP)) {
 			sigaddset(&t->pending.signal, SIGKILL);
 			signal_wake_up(t, 1);
-			nr++;
+			/* The vhost_worker does not particpate in coredumps */
+			if ((t->flags & (PF_USER_WORKER | PF_IO_WORKER)) != PF_USER_WORKER)
+				nr++;
 		}
 	}
 
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
@@ -29,7 +29,6 @@ struct kernel_clone_args {
 	u32 io_thread:1;
 	u32 user_worker:1;
 	u32 no_files:1;
-	u32 ignore_signals:1;
 	unsigned long stack;
 	unsigned long stack_size;
 	unsigned long tls;
diff --git a/include/linux/sched/vhost_task.h b/include/linux/sched/vhost_task.h
@@ -2,22 +2,13 @@
 #ifndef _LINUX_VHOST_TASK_H
 #define _LINUX_VHOST_TASK_H
 
-#include <linux/completion.h>
 
-struct task_struct;
+struct vhost_task;
 
-struct vhost_task {
-	int (*fn)(void *data);
-	void *data;
-	struct completion exited;
-	unsigned long flags;
-	struct task_struct *task;
-};
-
-struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
 				     const char *name);
 void vhost_task_start(struct vhost_task *vtsk);
 void vhost_task_stop(struct vhost_task *vtsk);
-bool vhost_task_should_stop(struct vhost_task *vtsk);
+void vhost_task_wake(struct vhost_task *vtsk);
 
 #endif
diff --git a/kernel/exit.c b/kernel/exit.c
@@ -411,7 +411,10 @@ static void coredump_task_exit(struct task_struct *tsk)
 	tsk->flags |= PF_POSTCOREDUMP;
 	core_state = tsk->signal->core_state;
 	spin_unlock_irq(&tsk->sighand->siglock);
-	if (core_state) {
+
+	/* The vhost_worker does not particpate in coredumps */
+	if (core_state &&
+	    ((tsk->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)) {
 		struct core_thread self;
 
 		self.task = current;
diff --git a/kernel/fork.c b/kernel/fork.c
@@ -2336,16 +2336,16 @@ __latent_entropy struct task_struct *copy_process(
 	p->flags &= ~PF_KTHREAD;
 	if (args->kthread)
 		p->flags |= PF_KTHREAD;
-	if (args->user_worker)
-		p->flags |= PF_USER_WORKER;
-	if (args->io_thread) {
+	if (args->user_worker) {
 		/*
-		 * Mark us an IO worker, and block any signal that isn't
+		 * Mark us a user worker, and block any signal that isn't
 		 * fatal or STOP
 		 */
-		p->flags |= PF_IO_WORKER;
+		p->flags |= PF_USER_WORKER;
 		siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
 	}
+	if (args->io_thread)
+		p->flags |= PF_IO_WORKER;
 
 	if (args->name)
 		strscpy_pad(p->comm, args->name, sizeof(p->comm));
@@ -2517,9 +2517,6 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cleanup_io;
 
-	if (args->ignore_signals)
-		ignore_signals(p);
-
 	stackleak_task_init(p);
 
 	if (pid != &init_struct_pid) {
diff --git a/kernel/signal.c b/kernel/signal.c
@@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p)
 
 	while_each_thread(p, t) {
 		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
-		count++;
+		/* Don't require de_thread to wait for the vhost_worker */
+		if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)
+			count++;
 
 		/* Don't bother with already dead threads */
 		if (t->exit_state)
@@ -2861,11 +2863,11 @@ bool get_signal(struct ksignal *ksig)
 		}
 
 		/*
-		 * PF_IO_WORKER threads will catch and exit on fatal signals
+		 * PF_USER_WORKER threads will catch and exit on fatal signals
 		 * themselves. They have cleanup that must be performed, so
 		 * we cannot call do_exit() on their behalf.
 		 */
-		if (current->flags & PF_IO_WORKER)
+		if (current->flags & PF_USER_WORKER)
 			goto out;
 
 		/*
diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
@@ -12,76 +12,106 @@ enum vhost_task_flags {
 	VHOST_TASK_FLAGS_STOP,
 };
 
+struct vhost_task {
+	bool (*fn)(void *data);
+	void *data;
+	struct completion exited;
+	unsigned long flags;
+	struct task_struct *task;
+};
+
 static int vhost_task_fn(void *data)
 {
 	struct vhost_task *vtsk = data;
-	int ret;
+	bool dead = false;
+
+	for (;;) {
+		bool did_work;
+
+		/* mb paired w/ vhost_task_stop */
+		if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))
+			break;
+
+		if (!dead && signal_pending(current)) {
+			struct ksignal ksig;
+			/*
+			 * Calling get_signal will block in SIGSTOP,
+			 * or clear fatal_signal_pending, but remember
+			 * what was set.
+			 *
+			 * This thread won't actually exit until all
+			 * of the file descriptors are closed, and
+			 * the release function is called.
+			 */
+			dead = get_signal(&ksig);
+			if (dead)
+				clear_thread_flag(TIF_SIGPENDING);
+		}
+
+		did_work = vtsk->fn(vtsk->data);
+		if (!did_work) {
+			set_current_state(TASK_INTERRUPTIBLE);
+			schedule();
+		}
+	}
 
-	ret = vtsk->fn(vtsk->data);
 	complete(&vtsk->exited);
-	do_exit(ret);
+	do_exit(0);
+}
+
+/**
+ * vhost_task_wake - wakeup the vhost_task
+ * @vtsk: vhost_task to wake
+ *
+ * wake up the vhost_task worker thread
+ */
+void vhost_task_wake(struct vhost_task *vtsk)
+{
+	wake_up_process(vtsk->task);
 }
+EXPORT_SYMBOL_GPL(vhost_task_wake);
 
 /**
  * vhost_task_stop - stop a vhost_task
  * @vtsk: vhost_task to stop
  *
- * Callers must call vhost_task_should_stop and return from their worker
- * function when it returns true;
+ * vhost_task_fn ensures the worker thread exits after
+ * VHOST_TASK_FLAGS_SOP becomes true.
  */
 void vhost_task_stop(struct vhost_task *vtsk)
 {
-	pid_t pid = vtsk->task->pid;
-
 	set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
-	wake_up_process(vtsk->task);
+	vhost_task_wake(vtsk);
 	/*
 	 * Make sure vhost_task_fn is no longer accessing the vhost_task before
-	 * freeing it below. If userspace crashed or exited without closing,
-	 * then the vhost_task->task could already be marked dead so
-	 * kernel_wait will return early.
+	 * freeing it below.
 	 */
 	wait_for_completion(&vtsk->exited);
-	/*
-	 * If we are just closing/removing a device and the parent process is
-	 * not exiting then reap the task.
-	 */
-	kernel_wait4(pid, NULL, __WCLONE, NULL);
 	kfree(vtsk);
 }
 EXPORT_SYMBOL_GPL(vhost_task_stop);
 
 /**
- * vhost_task_should_stop - should the vhost task return from the work function
- * @vtsk: vhost_task to stop
- */
-bool vhost_task_should_stop(struct vhost_task *vtsk)
-{
-	return test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags);
-}
-EXPORT_SYMBOL_GPL(vhost_task_should_stop);
-
-/**
- * vhost_task_create - create a copy of a process to be used by the kernel
- * @fn: thread stack
+ * vhost_task_create - create a copy of a task to be used by the kernel
+ * @fn: vhost worker function
  * @arg: data to be passed to fn
  * @name: the thread's name
  *
  * This returns a specialized task for use by the vhost layer or NULL on
  * failure. The returned task is inactive, and the caller must fire it up
  * through vhost_task_start().
  */
-struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg,
+struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg,
 				     const char *name)
 {
 	struct kernel_clone_args args = {
-		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM,
+		.flags		= CLONE_FS | CLONE_UNTRACED | CLONE_VM |
+				  CLONE_THREAD | CLONE_SIGHAND,
 		.exit_signal	= 0,
 		.fn		= vhost_task_fn,
 		.name		= name,
 		.user_worker	= 1,
 		.no_files	= 1,
-		.ignore_signals	= 1,
 	};
 	struct vhost_task *vtsk;
 	struct task_struct *tsk;

Original file line number	Diff line number	Diff line change
`@@ -39,7 +39,7 @@ extern void fpu_flush_thread(void);`
`39`	`39`	`static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)`
`40`	`40`	`{`
`41`	`41`	`if (cpu_feature_enabled(X86_FEATURE_FPU) &&`
`42`		`- !(current->flags & (PF_KTHREAD \| PF_IO_WORKER))) {`
	`42`	`+ !(current->flags & (PF_KTHREAD \| PF_USER_WORKER))) {`
`43`	`43`	`save_fpregs_to_fpstate(old_fpu);`
`44`	`44`	`/*`
`45`	`45`	`* The save operation preserved register state, so the`
Original file line number	Diff line number	Diff line change
`@@ -371,7 +371,9 @@ static int zap_process(struct task_struct *start, int exit_code)`
`371`	`371`	`if (t != current && !(t->flags & PF_POSTCOREDUMP)) {`
`372`	`372`	`sigaddset(&t->pending.signal, SIGKILL);`
`373`	`373`	`signal_wake_up(t, 1);`
`374`		`- nr++;`
	`374`	`+ /* The vhost_worker does not particpate in coredumps */`
	`375`	`+ if ((t->flags & (PF_USER_WORKER \| PF_IO_WORKER)) != PF_USER_WORKER)`
	`376`	`+ nr++;`
`375`	`377`	`}`
`376`	`378`	`}`
`377`	`379`