ext4: fix the might_sleep() warnings in kvfree()

Use the kvfree() in the RCU read critical section can trigger
the following warnings:

EXT4-fs (vdb): unmounting filesystem cd983e5b-3c83-4f5a-a136-17b00eb9d018.

WARNING: suspicious RCU usage

./include/linux/rcupdate.h:409 Illegal context switch in RCU read-side critical section!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1

Call Trace:
 <TASK>
 dump_stack_lvl+0xbb/0xd0
 dump_stack+0x14/0x20
 lockdep_rcu_suspicious+0x15a/0x1b0
 __might_resched+0x375/0x4d0
 ? put_object.part.0+0x2c/0x50
 __might_sleep+0x108/0x160
 vfree+0x58/0x910
 ? ext4_group_desc_free+0x27/0x270
 kvfree+0x23/0x40
 ext4_group_desc_free+0x111/0x270
 ext4_put_super+0x3c8/0xd40
 generic_shutdown_super+0x14c/0x4a0
 ? __pfx_shrinker_free+0x10/0x10
 kill_block_super+0x40/0x90
 ext4_kill_sb+0x6d/0xb0
 deactivate_locked_super+0xb4/0x180
 deactivate_super+0x7e/0xa0
 cleanup_mnt+0x296/0x3e0
 __cleanup_mnt+0x16/0x20
 task_work_run+0x157/0x250
 ? __pfx_task_work_run+0x10/0x10
 ? exit_to_user_mode_loop+0x6a/0x550
 exit_to_user_mode_loop+0x102/0x550
 do_syscall_64+0x44a/0x500
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

BUG: sleeping function called from invalid context at mm/vmalloc.c:3441
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 556, name: umount
preempt_count: 1, expected: 0
CPU: 3 UID: 0 PID: 556 Comm: umount
Call Trace:
 <TASK>
 dump_stack_lvl+0xbb/0xd0
 dump_stack+0x14/0x20
 __might_resched+0x275/0x4d0
 ? put_object.part.0+0x2c/0x50
 __might_sleep+0x108/0x160
 vfree+0x58/0x910
 ? ext4_group_desc_free+0x27/0x270
 kvfree+0x23/0x40
 ext4_group_desc_free+0x111/0x270
 ext4_put_super+0x3c8/0xd40
 generic_shutdown_super+0x14c/0x4a0
 ? __pfx_shrinker_free+0x10/0x10
 kill_block_super+0x40/0x90
 ext4_kill_sb+0x6d/0xb0
 deactivate_locked_super+0xb4/0x180
 deactivate_super+0x7e/0xa0
 cleanup_mnt+0x296/0x3e0
 __cleanup_mnt+0x16/0x20
 task_work_run+0x157/0x250
 ? __pfx_task_work_run+0x10/0x10
 ? exit_to_user_mode_loop+0x6a/0x550
 exit_to_user_mode_loop+0x102/0x550
 do_syscall_64+0x44a/0x500
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The above scenarios occur in initialization failures and teardown
paths, there are no parallel operations on the resources released
by kvfree(), this commit therefore remove rcu_read_lock/unlock() and
use rcu_access_pointer() instead of rcu_dereference() operations.

Fixes: 7c99072 ("ext4: fix potential race between s_flex_groups online resizing and access")
Fixes: df3da4e ("ext4: fix potential race between s_group_info online resizing and access")
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>
Link: https://patch.msgid.link/20260319094545.19291-1-qiang.zhang@linux.dev
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

Author: Zqiang

fs/ext4/mballoc.c

@@ -3584,9 +3584,7 @@ static int ext4_mb_init_backend(struct super_block *sb)
 	rcu_read_unlock();
 	iput(sbi->s_buddy_cache);
 err_freesgi:
-	rcu_read_lock();
-	kvfree(rcu_dereference(sbi->s_group_info));
-	rcu_read_unlock();
+	kvfree(rcu_access_pointer(sbi->s_group_info));
 	return -ENOMEM;
 }
 
@@ -3901,7 +3899,8 @@ void ext4_mb_release(struct super_block *sb)
 		WARN_ON_ONCE(!list_empty(&sbi->s_discard_list));
 	}
 
-	if (sbi->s_group_info) {
+	group_info = rcu_access_pointer(sbi->s_group_info);
+	if (group_info) {
 		for (i = 0; i < ngroups; i++) {
 			cond_resched();
 			grinfo = ext4_get_group_info(sb, i);
@@ -3919,12 +3918,9 @@ void ext4_mb_release(struct super_block *sb)
 		num_meta_group_infos = (ngroups +
 				EXT4_DESC_PER_BLOCK(sb) - 1) >>
 			EXT4_DESC_PER_BLOCK_BITS(sb);
-		rcu_read_lock();
-		group_info = rcu_dereference(sbi->s_group_info);
 		for (i = 0; i < num_meta_group_infos; i++)
 			kfree(group_info[i]);
 		kvfree(group_info);
-		rcu_read_unlock();
 	}
 	ext4_mb_avg_fragment_size_destroy(sbi);
 	ext4_mb_largest_free_orders_destroy(sbi);

fs/ext4/super.c

@@ -1254,27 +1254,23 @@ static void ext4_group_desc_free(struct ext4_sb_info *sbi)
 	struct buffer_head **group_desc;
 	int i;
 
-	rcu_read_lock();
-	group_desc = rcu_dereference(sbi->s_group_desc);
+	group_desc = rcu_access_pointer(sbi->s_group_desc);
 	for (i = 0; i < sbi->s_gdb_count; i++)
 		brelse(group_desc[i]);
 	kvfree(group_desc);
-	rcu_read_unlock();
 }
 
 static void ext4_flex_groups_free(struct ext4_sb_info *sbi)
 {
 	struct flex_groups **flex_groups;
 	int i;
 
-	rcu_read_lock();
-	flex_groups = rcu_dereference(sbi->s_flex_groups);
+	flex_groups = rcu_access_pointer(sbi->s_flex_groups);
 	if (flex_groups) {
 		for (i = 0; i < sbi->s_flex_groups_allocated; i++)
 			kvfree(flex_groups[i]);
 		kvfree(flex_groups);
 	}
-	rcu_read_unlock();
 }
 
 static void ext4_put_super(struct super_block *sb)