On Thu, Jul 09, 2015 at 07:45:45PM +0200, Lukas Czerner wrote: > Currently we can end up in a deadlock because of broken > sb_start_write -> s_umount ordering. > > The race goes like this: > > - write the file > - unlink the file - final_iput will not be calles as file is opened > - freeze the file system > - Now simultaneously close the file and call sync (or syncfs on that > particular file system). Sync will get to wait_sb_inodes() where it will > grab the referece to the inode (__iget()) and later to call iput().
This problem goes away with the sync scalability patchset that josef has been trying to get merged: git://git.kernel.org/pub/scm/linux/kernel/git/josef/btrfs-next.git superblock-scaling That patchset removes the full sb inodes list walk in wait_sb_inodes() and replaces it with a walk of inodes cleaned during the sync, which will be an empty list in the case of sync running on an empty filesystem. This commit does the work: https://git.kernel.org/cgit/linux/kernel/git/josef/btrfs-next.git/commit/?h=superblock-scaling&id=9bea30d5f4521db674203f365b1e0970588b2650 <As a separate note, can we *please* get that patchset merged given that there are now several outstanding issues that it fixes in one go?> > If we manage to close the file and drop the reference in between those > calls sync will attempt to do a iput_final() because the inode is now > unlinked and we're holding the last reference to it. This will > however block on a frozen file system (ext4_delete_inode for > example). > > Note that I've not been able to reproduce the issue, I've only seen this > happen once. However with some instrumentation (like msleep() in the > wait_sb_inodes() it can be achieved. > > Fix this by properly doing sb_start_write/sb_end_write to prevent us > from fsfreeze. > > Note that with this patch syncfs will block on the frozen file system > which is probably ok, but sync will block if any file system happens to > be frozen - not sure if that's a problem, but it's certainly different > from what we've been used to. sync should not block on frozen fileystems. By definition, a frozen filesystem is a clean filesystem, and so sync should really just be skipping over them. > +++ b/fs/super.c > @@ -514,10 +514,17 @@ void iterate_supers(void (*f)(struct super_block *, > void *), void *arg) > sb->s_count++; > spin_unlock(&sb_lock); > > + /* > + * Whatever we're going to do to the file system we have to > + * make sure that we'll not end up blocking on frozen file > + * system. > + */ > + sb_start_write(sb); > down_read(&sb->s_umount); > if (sb->s_root && (sb->s_flags & MS_BORN)) > f(sb, arg); > up_read(&sb->s_umount); > + sb_end_write(sb); > > spin_lock(&sb_lock); > if (p) That deadlocks sysrq-j (emergency thaw)... Cheers, Dave. -- Dave Chinner [email protected] -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
