Re: [PATCH] task_work: remove fifo ordering guarantee

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Aug 29, 2015 at 10:08:30AM -0700, Linus Torvalds wrote:
> Hmm.
> 
> I'm wondering if we should just make close_files() (or maybe even
> filp_close()) use a synchronous fput().
> 
> Iirc, the reason we delay fput() is that we had some nasty issues for
> the generic fput case. It was called from interrupt context by the aio
> code, and just in general there's a lot of nasty cases that can cause
> the final fput to happen (so there are lockdep issues with the mmap
> locks because the last fput being from munmap etc).
> 
> Maybe I forget some detail - it's been several years by now - but I
> think we could make the regular "close()" and "exit()" cases just use
> the synchronous fput (it's called "__fput_sync()" and currently
> explicitly limited to just kernel threads).
> 
> Al?

First of all, we'd better not count on e.g. delayed fput() *NOT* doing
task_work_add() - we still need to check if any new work had been added.
After all, final close() might very well have done a final mntput()
on a lazy-unmounted filesystem, possibly leaving us with fs shutdown via
task_work_add().  And if that sucker e.g. closes a socket, well, we are
back to closing an opened struct file, with task_work_add() etc.

I'm a bit nervious about filp_close() (that sucker is exported and widely
abused), but close_files()... sure, shouldn't be a problem.  And yes,
we can teach __close_fd() to do the same.  I really don't understand what's
the benefit, though - it's about the case when we are closing the last
descriptor for given opened file, so I would be rather surprised if slower
path taken on the way out to userland was not lost in noise...