Re: [PATCH] direct-io: use read lock for DIO_LOCKING flag

[Chao Yu <yuchao0@huawei.com>]

On 2021/4/16 8:43, Al Viro wrote:
> On Thu, Apr 15, 2021 at 12:24:13PM +0200, Jan Kara wrote:
>> On Thu 15-04-21 17:43:32, Chao Yu wrote:
>>> 9902af79c01a ("parallel lookups: actual switch to rwsem") changes inode
>>> lock from mutex to rwsem, however, we forgot to adjust lock for
>>> DIO_LOCKING flag in do_blockdev_direct_IO(),
> 
> The change in question had nothing to do with the use of ->i_mutex for
> regular files data access.
> 
>>> so let's change to hold read
>>> lock to mitigate performance regression in the case of read DIO vs read DIO,
>>> meanwhile it still keeps original functionality of avoiding buffered access
>>> vs direct access.
>>>
>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>>
>> Thanks for the patch but this is not safe. Originally we had exclusive lock
>> (with i_mutex), switching to rwsem doesn't change that requirement. It may
>> be OK for some filesystems to actually use shared acquisition of rwsem for
>> DIO reads but it is not clear that is fine for all filesystems (and I
>> suspect those filesystems that actually do care already don't use
>> DIO_LOCKING flag or were already converted to iomap_dio_rw()). So unless
>> you do audit of all filesystems using do_blockdev_direct_IO() with
>> DIO_LOCKING flag and make sure they are all fine with inode lock in shared
>> mode, this is a no-go.
> 
> Aye.  Frankly, I would expect that anyone bothering with that kind of
> analysis for given filesystem (and there are fairly unpleasant ones in the
> list) would just use the fruits of those efforts to convert it over to
> iomap.

Actually, I was misguided by DIO_LOCKING comments more or less, it looks it
was introduced to avoid race case only in between buffered IO and DIO.

	/* need locking between buffered and direct access */
	DIO_LOCKING	= 0x01,

I don't think it is easy for me to analyse usage scenario/restriction of all
DIO_LOCKING users, and get their developers' acks for this change.

Converting fs to use iomap_dio_rw looks a better option for me, thanks, Jan
and Al. :)

Thanks,

> 
> "Read DIO" does not mean that accesses to private in-core data structures used
> by given filesystem can be safely done in parallel.  So blanket patch like
> that is not safe at all.
> .
>