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When the user mounts the erofs second times, the decompression thread
may hung. The problem happens due to a sequence of steps like the
following:

1) Task A called z_erofs_load_lzma_config which obtain all of the node
   from the z_erofs_lzma_head.

2) At this time, task B called the z_erofs_lzma_decompress and wanted to
   get a node. But the z_erofs_lzma_head was empty, the Task B had to
   sleep.

3) Task A release nodes and push nodes into the z_erofs_lzma_head. But
   task B was still sleeping.

One example report when the hung happens:
task:kworker/u3:1 state:D stack:14384 pid: 86 ppid: 2 flags:0x00004000
Workqueue: erofs_unzipd z_erofs_decompressqueue_work
Call Trace:
 <TASK>
 __schedule+0x281/0x760
 schedule+0x49/0xb0
 z_erofs_lzma_decompress+0x4bc/0x580
 ? cpu_core_flags+0x10/0x10
 z_erofs_decompress_pcluster+0x49b/0xba0
 ? __update_load_avg_se+0x2b0/0x330
 ? __update_load_avg_se+0x2b0/0x330
 ? update_load_avg+0x5f/0x690
 ? update_load_avg+0x5f/0x690
 ? set_next_entity+0xbd/0x110
 ? _raw_spin_unlock+0xd/0x20
 z_erofs_decompress_queue.isra.0+0x2e/0x50
 z_erofs_decompressqueue_work+0x30/0x60
 process_one_work+0x1d3/0x3a0
 worker_thread+0x45/0x3a0
 ? process_one_work+0x3a0/0x3a0
 kthread+0xe2/0x110
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x22/0x30
 </TASK>
Signed-off-by: Yuwen Chen <chenyuwen1@meizu.com>
Fixes: 622ceadd

 ("erofs: lzma compression support")
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220626224041.4288-1-chenyuwen1@meizu.com

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>