Re: [PATCH] add mutex to fbdev for fb_mmap locking

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 2 Jun 2009, Alan Cox wrote:
>
> > ie the fact that the TTY layer does user-mode copies while holding some 
> > tty lock. So now the tty layer introduces that chain from some random lock 
> > to the mmap_sem.
> 
> Its basically holding the termios lock to copy from the struct termios to
> user space which means its trivial to do copy to a stack buffer first. I
> can fix that pretty easily if you want.

It would be good. I don't know if it matters for any other path, but 
mmap_sem has always been a total _bitch_ to work around for deadlocks, so 
it's always good to try to avoid holding another lock while doing copying 
to/from user. 

I thought we already always copied things to a buffer (for conversion 
reasons, ie doing the whole "ktermios<->random-user-termios-of-the-day" 
thing), but I guess I was wrong.

		Linus

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