[MDEV-24094] Race condition and hang in INFORMATION_SCHEMA.INNODB_TABLESPACES_ENCRYPTION Created: 2020-11-02  Updated: 2020-11-02  Resolved: 2020-11-02

Status: Closed
Project: MariaDB Server
Component/s: Information Schema, Storage Engine - InnoDB
Affects Version/s: 10.5.7
Fix Version/s: N/A

Type: Bug Priority: Major
Reporter: Marko Mäkelä Assignee: Marko Mäkelä
Resolution: Not a Bug Votes: 0
Labels: regression


 Description   

The reduction of fil_system.mutex contention introduced a race condition that can lead to a hang of the server when the view INFORMATION_SCHEMA.INNODB_TABLESPACES_ENCRYPTION is accessed. If space->is_stopping() starts to hold right after the check, then we would attempt to acquire fil_system.mutex while we are already holding it.

The fix is simple (and also clarifies what this is about):

diff --git a/storage/innobase/handler/i_s.cc b/storage/innobase/handler/i_s.cc
 
index ecc0905de56..4c9098f9a86 100644
 
--- a/storage/innobase/handler/i_s.cc
 
+++ b/storage/innobase/handler/i_s.cc
 
@@ -7055,8 +7055,7 @@ i_s_tablespaces_encryption_fill_table(
 
 	for (fil_space_t* space = UT_LIST_GET_FIRST(fil_system.space_list);
 
 	     space; space = UT_LIST_GET_NEXT(space_list, space)) {
 
 		if (space->purpose == FIL_TYPE_TABLESPACE
 
-		    && !space->is_stopping()) {
 
-			space->reacquire();
 
+		    && space->acquire_if_not_stopped(true)) {
 
 			mutex_exit(&fil_system.mutex);
 
 			if (int err = i_s_dict_fill_tablespaces_encryption(
 
 				    thd, space, tables->table)) {

To avoid the race condition, we must atomically check the is_stopping() flag while incrementing the reference count.

 Comments   
Comment by Marko Mäkelä [ 2020-11-02 ]

This is not a bug after all. I encountered this while rebasing MDEV-21452 to the latest 10.5. While resolving conflicts, I replaced space->reacquire() above with space->acquire(), which would attempt to open the file if it was closed. It was not even a race condition: the space->is_open() is a separate flag from space->is_stopping().

This bug only existed in my local repository and was never pushed anywhere. I am sorry for the noise.

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