[PATCH v4 04/11] liveupdate: Protect FLB lists with luo_register_rwlock

[Pasha Tatashin]

Because liveupdate FLB objects will soon drop their persistent module
references when registered, list traversals must be protected against
concurrent module unloading.

To provide this protection, utilize the global luo_register_rwlock.
It protects the global registry of FLBs and the handler's specific
list of FLB dependencies.

Read locks are used during concurrent list traversals (e.g., during
preservation and serialization). Write locks are taken during registration
and unregistration.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Reviewed-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>
---
 include/linux/liveupdate.h  |  1 +
 kernel/liveupdate/luo_flb.c | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/liveupdate.h b/include/linux/liveupdate.h
index 61325ad26526..9c761d9bacf8 100644
--- a/include/linux/liveupdate.h
+++ b/include/linux/liveupdate.h
@@ -12,6 +12,7 @@
 #include <linux/kho/abi/luo.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/types.h>
 #include <uapi/linux/liveupdate.h>
 
diff --git a/kernel/liveupdate/luo_flb.c b/kernel/liveupdate/luo_flb.c
index cf4a8f854c83..fdb274410e8f 100644
--- a/kernel/liveupdate/luo_flb.c
+++ b/kernel/liveupdate/luo_flb.c
@@ -245,17 +245,20 @@ int luo_flb_file_preserve(struct liveupdate_file_handler *fh)
 	struct luo_flb_link *iter;
 	int err = 0;
 
+	down_read(&luo_register_rwlock);
 	list_for_each_entry(iter, flb_list, list) {
 		err = luo_flb_file_preserve_one(iter->flb);
 		if (err)
 			goto exit_err;
 	}
+	up_read(&luo_register_rwlock);
 
 	return 0;
 
 exit_err:
 	list_for_each_entry_continue_reverse(iter, flb_list, list)
 		luo_flb_file_unpreserve_one(iter->flb);
+	up_read(&luo_register_rwlock);
 
 	return err;
 }
@@ -277,6 +280,7 @@ void luo_flb_file_unpreserve(struct liveupdate_file_handler *fh)
 	struct list_head *flb_list = &ACCESS_PRIVATE(fh, flb_list);
 	struct luo_flb_link *iter;
 
+	guard(rwsem_read)(&luo_register_rwlock);
 	list_for_each_entry_reverse(iter, flb_list, list)
 		luo_flb_file_unpreserve_one(iter->flb);
 }
@@ -297,6 +301,7 @@ void luo_flb_file_finish(struct liveupdate_file_handler *fh)
 	struct list_head *flb_list = &ACCESS_PRIVATE(fh, flb_list);
 	struct luo_flb_link *iter;
 
+	guard(rwsem_read)(&luo_register_rwlock);
 	list_for_each_entry_reverse(iter, flb_list, list)
 		luo_flb_file_finish_one(iter->flb);
 }
@@ -360,6 +365,8 @@ int liveupdate_register_flb(struct liveupdate_file_handler *fh,
 	if (!luo_session_quiesce())
 		return -EBUSY;
 
+	down_write(&luo_register_rwlock);
+
 	/* Check that this FLB is not already linked to this file handler */
 	err = -EEXIST;
 	list_for_each_entry(iter, flb_list, list) {
@@ -401,11 +408,13 @@ int liveupdate_register_flb(struct liveupdate_file_handler *fh,
 	private->users++;
 	link->flb = flb;
 	list_add_tail(&no_free_ptr(link)->list, flb_list);
+	up_write(&luo_register_rwlock);
 	luo_session_resume();
 
 	return 0;
 
 err_resume:
+	up_write(&luo_register_rwlock);
 	luo_session_resume();
 	return err;
 }
@@ -449,6 +458,8 @@ int liveupdate_unregister_flb(struct liveupdate_file_handler *fh,
 	if (!luo_session_quiesce())
 		return -EBUSY;
 
+	down_write(&luo_register_rwlock);
+
 	/* Find and remove the link from the file handler's list */
 	list_for_each_entry(iter, flb_list, list) {
 		if (iter->flb == flb) {
@@ -473,11 +484,13 @@ int liveupdate_unregister_flb(struct liveupdate_file_handler *fh,
 		module_put(flb->ops->owner);
 	}
 
+	up_write(&luo_register_rwlock);
 	luo_session_resume();
 
 	return 0;
 
 err_resume:
+	up_write(&luo_register_rwlock);
 	luo_session_resume();
 	return err;
 }
@@ -643,6 +656,7 @@ void luo_flb_serialize(void)
 	struct liveupdate_flb *gflb;
 	int i = 0;
 
+	guard(rwsem_read)(&luo_register_rwlock);
 	list_private_for_each_entry(gflb, &luo_flb_global.list, private.list) {
 		struct luo_flb_private *private = luo_flb_get_private(gflb);
 
-- 
2.43.0