erofs: fix pcluster use-after-free on UP platforms

During stress testing with CONFIG_SMP disabled, KASAN reports as below:

==================================================================
BUG: KASAN: use-after-free in __mutex_lock+0xe5/0xc30
Read of size 8 at addr ffff8881094223f8 by task stress/7789

CPU: 0 PID: 7789 Comm: stress Not tainted 6.0.0-rc1-00002-g0d53d2e882f9 #3
Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
Call Trace:
 <TASK>
..
 __mutex_lock+0xe5/0xc30
..
 z_erofs_do_read_page+0x8ce/0x1560
..
 z_erofs_readahead+0x31c/0x580
..
Freed by task 7787
 kasan_save_stack+0x1e/0x40
 kasan_set_track+0x20/0x30
 kasan_set_free_info+0x20/0x40
 __kasan_slab_free+0x10c/0x190
 kmem_cache_free+0xed/0x380
 rcu_core+0x3d5/0xc90
 __do_softirq+0x12d/0x389

Last potentially related work creation:
 kasan_save_stack+0x1e/0x40
 __kasan_record_aux_stack+0x97/0xb0
 call_rcu+0x3d/0x3f0
 erofs_shrink_workstation+0x11f/0x210
 erofs_shrink_scan+0xdc/0x170
 shrink_slab.constprop.0+0x296/0x530
 drop_slab+0x1c/0x70
 drop_caches_sysctl_handler+0x70/0x80
 proc_sys_call_handler+0x20a/0x2f0
 vfs_write+0x555/0x6c0
 ksys_write+0xbe/0x160
 do_syscall_64+0x3b/0x90

The root cause is that erofs_workgroup_unfreeze() doesn't reset to
orig_val thus it causes a race that the pcluster reuses unexpectedly
before freeing.

Since UP platforms are quite rare now, such path becomes unnecessary.
Let's drop such specific-designed path directly instead.

Fixes: 73f5c66d ("staging: erofs: fix `erofs_workgroup_{try_to_freeze, unfreeze}'")
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220902045710.109530-1-hsiangkao@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/internal.h

@@ -195,7 +195,6 @@ struct erofs_workgroup {
 	atomic_t refcount;
 };
 
-#if defined(CONFIG_SMP)
 static inline bool erofs_workgroup_try_to_freeze(struct erofs_workgroup *grp,
 						 int val)
 {
@@ -224,34 +223,6 @@ static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
 	return atomic_cond_read_relaxed(&grp->refcount,
 					VAL != EROFS_LOCKED_MAGIC);
 }
-#else
-static inline bool erofs_workgroup_try_to_freeze(struct erofs_workgroup *grp,
-						 int val)
-{
-	preempt_disable();
-	/* no need to spin on UP platforms, let's just disable preemption. */
-	if (val != atomic_read(&grp->refcount)) {
-		preempt_enable();
-		return false;
-	}
-	return true;
-}
-
-static inline void erofs_workgroup_unfreeze(struct erofs_workgroup *grp,
-					    int orig_val)
-{
-	preempt_enable();
-}
-
-static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
-{
-	int v = atomic_read(&grp->refcount);
-
-	/* workgroup is never freezed on uniprocessor systems */
-	DBG_BUGON(v == EROFS_LOCKED_MAGIC);
-	return v;
-}
-#endif	/* !CONFIG_SMP */
 #endif	/* !CONFIG_EROFS_FS_ZIP */
 
 /* we strictly follow PAGE_SIZE and no buffer head yet */