Messages in this thread | | | Subject | Re: [PATCH] direct-io: use read lock for DIO_LOCKING flag | From | Chao Yu <> | Date | Fri, 16 Apr 2021 10:19:27 +0800 |
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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. > . >
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