On Fri 10-07-15 09:40:12, Dave Chinner wrote: > 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?> Not sure where that got stuck - oh, maybe on Tejun's memcg writeback series which was clashing with it. Josef? > > 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. Just for record I agree with Dave. Sync on frozen fs should just return. And freeze protection in iterate_supers() looks just wrong. Honza > > +++ 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 > david@xxxxxxxxxxxxx > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
