Messages in this thread | | | Subject | Re: [PATCH] writeback_inodes can race with unmount | From | Chris Mason <> | Date | Tue, 08 Jun 2004 21:29:26 -0400 |
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On Tue, 2004-06-08 at 17:56, Andrew Morton wrote: > Chris Mason <mason@suse.com> wrote:
> > > Why don't we have the same race in the sync() path as well? Moving the > > > locking to sync_sb_inodes() itself would fix it also. > > > > In the sync() path we're already taking a read lock on s_umount sem. > > Moving the locking into sync_sb_inodes would be tricky, it is sometimes > > called with the write lock on s_umount_sem held and sometimes with a > > read lock. > > > > Plus we'd be dead if we had to do the above. If that read_trylock() fails > the sync() will forget to sync stuff. > > And, contra your description, we'll fail the trylock relatively frequently > - when some other process is writing back this superblock.
The write lock is taken much less frequently. Should be just on mount/unmount no?
> > But as this codepath is never used for data-integrity writeout things > should be OK. I'll slip a comment in there and maybe a > BUG_ON(wbc->sync_mode == WB_SYNC_ALL). > > You didn't tell us where the window is? It's a bit weird that some other > process can come in and unmount the fs while there are non-zero-refcount > inodes floating about on the superblock.
It's the iput in sync_sb_inodes(). The test workload is 8 parallel runs of this loop (each run to a different /dev/xxx)
while(true) ; do mkfs /dev/xxx mount /dev/xxx /xxx dd if=/dev/zero of=/xxx/foo bs=1MB count=80 rm /xxx/foo umount /dev/xxx done
So, each FS only has a single dirty inode, and that single inode gets deleted during the run. With enough memory pressure, write calls frequently go into throttling, increasing the chance for sync_sb_inodes to be called for writeback at the same time as unmount.
Right before the iput in sync_sb_inodes, we've dropped both the sb_lock and the inode_lock, nothing prevents the unmount from continuing. The super block won't be freed because we've got it pinned, but the unmount can proceed.
So, it looks like we get this:
CPU 0 CPU 1 writeback_inodes generic_shutdown_super sync_sb_inodes iget(inode) spin_unlock(&inode_lock) sop->put_super(sb) iput(inode) generic_delete_inode()
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