From 8ccb6e8d12e84d84e4b86be38253c8c91469c653 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Thu, 20 Jan 2022 23:24:42 +0000
Subject: [PATCH 35/37] autoconf(9): New function config_detach_commit.
When a driver's .ca_detach function has committed to detaching -- it
definitely won't back out with EBUSY, for instance -- it can call
this to wake all pending calls to device_lookup_acquire and make them
fail immediately.
This is necessary to break a deadlock if the device_lookup_acquire
calls happen inside I/O operations which the driver's .ca_detach
function waits for the completion of -- without config_detach_commit,
I/O operations would be stuck in device_lookup_acquire waiting for
.ca_detach and .ca_detach would be stuck waiting for I/O operations
to return.
Most drivers won't need to call this: for autoconf drivers used the
traditional way by devsw for userland device nodes, the .ca_detach
routine uses vdevgone, and we will arrange to make vdevgone call
config_detach_commit automagically in such drivers anyway.
XXX kernel ABI change to struct device requires bump -- later change
will make struct device opaque to ABI, but we're not there yet
---
sys/kern/subr_autoconf.c | 45 ++++++++++++++++++++++++++++++++++------
sys/sys/device.h | 2 ++
2 files changed, 41 insertions(+), 6 deletions(-)
diff --git a/sys/kern/subr_autoconf.c b/sys/kern/subr_autoconf.c
index 74f71dca0bcf..03afc7f54b55 100644
--- a/sys/kern/subr_autoconf.c
+++ b/sys/kern/subr_autoconf.c
@@ -2045,10 +2045,17 @@ config_detach(device_t dev, int flags)
* If it was possible to detach the device, ensure that the
* device is deactivated.
*/
- if (rv == 0)
- dev->dv_flags &= ~DVF_ACTIVE;
- else if ((flags & DETACH_FORCE) == 0) {
- /* Detach failed -- likely EBUSY. */
+ if (rv == 0) {
+ config_detach_commit(dev);
+ dev->dv_flags &= ~DVF_ACTIVE; /* XXXSMP */
+ } else if ((flags & DETACH_FORCE) == 0) {
+ /*
+ * Detach failed -- likely EBUSY. Driver must not have
+ * called config_detach_commit.
+ */
+ KASSERTMSG(!dev->dv_detached,
+ "%s committed to detaching and then backed out",
+ device_xname(dev));
goto out;
} else {
panic("config_detach: forced detach of %s failed (%d)",
@@ -2065,7 +2072,8 @@ config_detach(device_t dev, int flags)
* responsibility of .ca_detach to ensure anything with open
* references will be interrupted and release them promptly,
* not block indefinitely. All new attempts to acquire
- * references will block until dv_detaching clears.
+ * references will fail, as config_detach_commit has arranged
+ * by now.
*/
mutex_enter(&config_misc_lock);
localcount_drain(dev->dv_localcount,
@@ -2139,6 +2147,30 @@ out:
return rv;
}
+/*
+ * config_detach_commit(dev)
+ *
+ * Issued by a driver's .ca_detach routine to notify anyone
+ * waiting in device_lookup_acquire that the driver is committed
+ * to detaching the device, which allows device_lookup_acquire to
+ * wake up and fail immediately.
+ *
+ * Safe to call multiple times -- idempotent. Must be called
+ * during config_detach_enter/exit. Safe to use with
+ * device_lookup because the device is not actually removed from
+ * the table until after config_detach_exit.
+ */
+void
+config_detach_commit(device_t dev)
+{
+
+ mutex_enter(&config_misc_lock);
+ KASSERT(dev->dv_detaching == curlwp);
+ dev->dv_detached = true;
+ cv_broadcast(&config_misc_cv);
+ mutex_exit(&config_misc_lock);
+}
+
int
config_detach_children(device_t parent, int flags)
{
@@ -2646,7 +2678,8 @@ device_lookup_acquire(cfdriver_t cd, int unit)
mutex_enter(&alldevs_lock);
retry: if (unit < 0 || unit >= cd->cd_ndevs ||
(dv = cd->cd_devs[unit]) == NULL ||
- dv->dv_del_gen != 0) {
+ dv->dv_del_gen != 0 ||
+ dv->dv_detached) {
dv = NULL;
} else {
/*
diff --git a/sys/sys/device.h b/sys/sys/device.h
index 9f95c98ffa42..cfd324a1ac2d 100644
--- a/sys/sys/device.h
+++ b/sys/sys/device.h
@@ -281,6 +281,7 @@ struct device {
struct lwp *dv_attaching; /* thread not yet finished in attach */
struct lwp *dv_detaching; /* detach lock (config_misc_lock/cv) */
+ bool dv_detached; /* config_misc_lock */
size_t dv_activity_count;
void (**dv_activity_handlers)(device_t, devactive_t);
@@ -631,6 +632,7 @@ device_t config_attach_pseudo(cfdata_t);
int config_detach(device_t, int);
int config_detach_children(device_t, int flags);
+void config_detach_commit(device_t);
bool config_detach_all(int);
int config_deactivate(device_t);
void config_defer(device_t, void (*)(device_t));