[Devel] Re: [PATCH] incorrect error handling inside generic_file_direct_write

[Andrew Morton]

On Tue, 12 Dec 2006 16:18:32 +0300
Dmitriy Monakhov <dmonakhov at sw.ru> wrote:

> >> but according to filemaps locking rules: mm/filemap.c:77
> >>  ..
> >>  *  ->i_mutex			(generic_file_buffered_write)
> >>  *    ->mmap_sem		(fault_in_pages_readable->do_page_fault)
> >>  ..
> >> I'm confused a litle bit, where is the truth? 
> >
> > xfs_write() calls generic_file_direct_write() without taking i_mutex for
> > O_DIRECT writes.
> Yes, but my quastion is about __generic_file_aio_write_nolock().
> As i understand _nolock sufix means that i_mutex was already locked 
> by caller, am i right ?

Nope.  It just means that __generic_file_aio_write_nolock() doesn't take
the lock.  We don't assume or require that the caller took it.  For example
the raw driver calls generic_file_aio_write_nolock() without taking
i_mutex.  Raw isn't relevant to the problem (although ocfs2 might be).  But
we cannot assume that all callers have taken i_mutex, I think.

I guess we can make that a rule (document it, add
BUG_ON(!mutex_is_locked(..)) if it isn't a blockdev) if needs be.  After
really checking that this matches reality for all callers.

It's important, too - if we have an unprotected i_size_write() then the
seqlock can get out of sync due to a race and then i_size_read() locks up
the kernel.