On Wed, Oct 23, 2019 at 12:01:53PM +0200, Jan Kara wrote:
> On Wed 23-10-19 13:35:19, Matthew Bobrowski wrote:
> > On Mon, Oct 21, 2019 at 09:43:30PM +0200, Jan Kara wrote:
> > > On Mon 21-10-19 09:31:12, Theodore Y. Ts'o wrote:
> > > > Hi Matthew, thanks for your work on this patch series!
> > > >
> > > > I applied it against 4c3, and ran a quick test run on it, and found
> > > > the following locking problem. To reproduce:
> > > >
> > > > kvm-xfstests -c nojournal generic/113
> > > >
> > > > generic/113 [09:27:19][ 5.841937] run fstests generic/113 at 2019-10-21 09:27:19
> > > > [ 7.959477]
> > > > [ 7.959798] ============================================
> > > > [ 7.960518] WARNING: possible recursive locking detected
> > > > [ 7.961225] 5.4.0-rc3-xfstests-00012-g7fe6ea084e48 #1238 Not tainted
> > > > [ 7.961991] --------------------------------------------
> > > > [ 7.962569] aio-stress/1516 is trying to acquire lock:
> > > > [ 7.963129] ffff9fd4791148c8 (&sb->s_type->i_mutex_key#12){++++}, at: __generic_file_fsync+0x3e/0xb0
> > > > [ 7.964109]
> > > > [ 7.964109] but task is already holding lock:
> > > > [ 7.964740] ffff9fd4791148c8 (&sb->s_type->i_mutex_key#12){++++}, at: ext4_dio_write_iter+0x15b/0x430
> > >
> > > This is going to be a tricky one. With iomap, the inode locking is handled
> > > by the filesystem while calling generic_write_sync() is done by
> > > iomap_dio_rw(). I would really prefer to avoid tweaking iomap_dio_rw() not
> > > to call generic_write_sync(). So we need to remove inode_lock from
> > > __generic_file_fsync() (used from ext4_sync_file()). This locking is mostly
> > > for legacy purposes and we don't need this in ext4 AFAICT - but removing
> > > the lock from __generic_file_fsync() would mean auditing all legacy
> > > filesystems that use this to make sure flushing inode & its metadata buffer
> > > list while it is possibly changing cannot result in something unexpected. I
> > > don't want to clutter this series with it so we are left with
> > > reimplementing __generic_file_fsync() inside ext4 without inode_lock. Not
> > > too bad but not great either. Thoughts?
> >
> > So, I just looked at this on my lunch break and I think the simplest
> > approach would be to just transfer the necessary chunks of code from
> > within __generic_file_fsync() into ext4_sync_file() for !journal cases,
> > minus the inode lock, and minus calling into __generic_file_fsync(). I
> > don't forsee this causing any issues, but feel free to correct me if I'm
> > wrong.
>
> Yes, that's what I'd suggest as well. In fact when doing that you can share
> file_write_and_wait_range() call with the one already in ext4_sync_file()
> use for other cases. Similarly with file_check_and_advance_wb_err(). So the
> copied bit will be really only:
>
> ret = sync_mapping_buffers(inode->i_mapping);
> if (!(inode->i_state & I_DIRTY_ALL))
> goto out;
> if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
> goto out;
>
> err = sync_inode_metadata(inode, 1);
> if (ret == 0)
> ret = err;
>
> > If this is deemed to be OK, then I will go ahead and include this as a
> > separate patch in my series.
>
> Yes, please.
Heh!
I just finished writing and testing it and exactly what I've done
(attached). Anyway, I will include it in v6. :)
--<M>--
>From 4c82edb34324f91788c941956954d4e7e1886c2c Mon Sep 17 00:00:00 2001
From: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>
Date: Wed, 23 Oct 2019 17:43:23 +1100
Subject: [PATCH 1/2] ext4: update ext4_sync_file() to not use
__generic_file_fsync()
When the filesystem is created without a journal, we eventually call
into __generic_file_fsync() in order to write out all the modified
in-core data to the permanent storage device. This function happens to
try and obtain an inode_lock() while synchronizing the files buffer
and it's associated metadata.
Generally, this is fine, however it becomes a problem when there is
higher level code that has already obtained an inode_lock() as this
leads to a recursive lock situation. This case is especially true when
porting across direct I/O to iomap infrastructure as we obtain an
inode_lock() early on in the I/O within ext4_dio_write_iter() and hold
it until the I/O has been completed. Consequently, to not run into
this specific issue, we move away from calling into
__generic_file_fsync() and perform the necessary synchronization tasks
within ext4_sync_file().
Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx>
---
fs/ext4/fsync.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index 5508baa11bb6..9e11868e82f9 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -116,8 +116,21 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
goto out;
}
+ ret = file_write_and_wait_range(file, start, end);
+ if (ret)
+ return ret;
+
if (!journal) {
- ret = __generic_file_fsync(file, start, end, datasync);
+ ret = sync_mapping_buffers(inode->i_mapping);
+ if (!(inode->i_state & I_DIRTY_ALL))
+ goto out;
+ if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
+ goto out;
+
+ err = sync_inode_metadata(inode, 1);
+ if (!ret)
+ ret = err;
+
if (!ret)
ret = ext4_sync_parent(inode);
if (test_opt(inode->i_sb, BARRIER))
@@ -125,9 +138,6 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
goto out;
}
- ret = file_write_and_wait_range(file, start, end);
- if (ret)
- return ret;
/*
* data=writeback,ordered:
* The caller's filemap_fdatawrite()/wait will sync the data.
--
2.20.1
