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H A Draid5-cache.cdiff b13015af94cf405f73ff64ce0797269554020c37 Wed Jun 08 18:27:52 CEST 2022 Logan Gunthorpe <logang@deltatee.com> md/raid5-cache: Clear conf->log after finishing work

A NULL pointer dereferlence on conf->log is seen randomly with
the mdadm test 21raid5cache. Kasan reporst:

BUG: KASAN: null-ptr-deref in r5l_reclaimable_space+0xf5/0x140
Read of size 8 at addr 0000000000000860 by task md0_reclaim/3086

Call Trace:
dump_stack_lvl+0x5a/0x74
kasan_report.cold+0x5f/0x1a9
__asan_load8+0x69/0x90
r5l_reclaimable_space+0xf5/0x140
r5l_do_reclaim+0xf4/0x5e0
r5l_reclaim_thread+0x69/0x3b0
md_thread+0x1a2/0x2c0
kthread+0x177/0x1b0
ret_from_fork+0x22/0x30

This is caused by conf->log being cleared in r5l_exit_log() before
stopping the reclaim thread.

To fix this, clear conf->log after the reclaim_thread is unregistered
and after flushing disable_writeback_work.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>