Re: [PATCH v2] hfsplus: remove mutex_lock check in hfsplus_free_extents

[Yangtao Li <frank.li@vivo.com>]

Hi Slava，

在 2025/5/30 02:34, Viacheslav Dubeyko 写道:
> On Thu, 2025-05-29 at 00:18 -0600, Yangtao Li wrote:
>> Syzbot reported an issue in hfsplus filesystem:
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 4400 at fs/hfsplus/extents.c:346
>> 	hfsplus_free_extents+0x700/0xad0
>> Call Trace:
>> <TASK>
>> hfsplus_file_truncate+0x768/0xbb0 fs/hfsplus/extents.c:606
>> hfsplus_write_begin+0xc2/0xd0 fs/hfsplus/inode.c:56
>> cont_expand_zero fs/buffer.c:2383 [inline]
>> cont_write_begin+0x2cf/0x860 fs/buffer.c:2446
>> hfsplus_write_begin+0x86/0xd0 fs/hfsplus/inode.c:52
>> generic_cont_expand_simple+0x151/0x250 fs/buffer.c:2347
>> hfsplus_setattr+0x168/0x280 fs/hfsplus/inode.c:263
>> notify_change+0xe38/0x10f0 fs/attr.c:420
>> do_truncate+0x1fb/0x2e0 fs/open.c:65
>> do_sys_ftruncate+0x2eb/0x380 fs/open.c:193
>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>> do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
>> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> To avoid deadlock, Commit 31651c607151 ("hfsplus: avoid deadlock
>> on file truncation") unlock extree before hfsplus_free_extents(),
>> and add check wheather extree is locked in hfsplus_free_extents().
>>
>> However, when operations such as hfsplus_file_release,
>> hfsplus_setattr, hfsplus_unlink, and hfsplus_get_block are executed
>> concurrently in different files, it is very likely to trigger the
>> WARN_ON, which will lead syzbot and xfstest to consider it as an
>> abnormality.
>>
>> The comment above this warning also describes one of the easy
>> triggering situations, which can easily trigger and cause
>> xfstest&syzbot to report errors.
>>
>> [task A]			[task B]
>> ->hfsplus_file_release
>>    ->hfsplus_file_truncate
>>      ->hfs_find_init
>>        ->mutex_lock
>>      ->mutex_unlock
>> 				->hfsplus_write_begin
>> 				  ->hfsplus_get_block
>> 				    ->hfsplus_file_extend
>> 				      ->hfsplus_ext_read_extent
>> 				        ->hfs_find_init
>> 					  ->mutex_lock
>>      ->hfsplus_free_extents
>>        WARN_ON(mutex_is_locked) !!!
>>
>> Several threads could try to lock the shared extents tree.
>> And warning can be triggered in one thread when another thread
>> has locked the tree. This is the wrong behavior of the code and
>> we need to remove the warning.
>>
>> Fixes: 31651c607151f ("hfsplus: avoid deadlock on file truncation")
>> Reported-by: syzbot+8c0bc9f818702ff75b76@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/00000000000057fa4605ef101c4c@google.com/
>> Signed-off-by: Yangtao Li <frank.li@vivo.com>
>> ---
>>   fs/hfsplus/extents.c | 3 ---
>>   1 file changed, 3 deletions(-)
>>
>> diff --git a/fs/hfsplus/extents.c b/fs/hfsplus/extents.c
>> index a6d61685ae79..b1699b3c246a 100644
>> --- a/fs/hfsplus/extents.c
>> +++ b/fs/hfsplus/extents.c
>> @@ -342,9 +342,6 @@ static int hfsplus_free_extents(struct super_block *sb,
>>   	int i;
>>   	int err = 0;
>>   
>> -	/* Mapping the allocation file may lock the extent tree */
>> -	WARN_ON(mutex_is_locked(&HFSPLUS_SB(sb)->ext_tree->tree_lock));
>> -
> 
> Makes sense to me. Looks good.
> 
> But I really like your mentioning of reproducing the issue in generic/013 and
> really nice analysis of the issue there. Sadly, we haven't it in the comment. :)

Oh, so this is what you mean, but the description is already very long. 
I think this may be enough...

> 
> Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
> 
> Thanks,
> Slava.
> 
>>   	hfsplus_dump_extent(extent);
>>   	for (i = 0; i < 8; extent++, i++) {
>>   		count = be32_to_cpu(extent->block_count);
> 

Thx，
Yangtao