Re: [06/11] tty: fix race in tty_fasync

[Linus Torvalds]

On Tue, 26 Jan 2010, Eric W. Biederman wrote:

> Greg KH <gregkh@xxxxxxx> writes:
> 
> > 2.6.27-stable review patch.  If anyone has any objections, please let us know.
> 
> Only that __f_setown by way of f_modown unconditionally enables interrupts.  So
> without touching f_modown as well in mainline we have nasty sounding lockdep warnings.

Hmm. That seems to be true in mainline too, isn't it?

So now we have:
 - tty_fasync() gets tty->ctrl_lock, with spin_lock_irqsave()

 - it then calls __f_setown()

 - which calls f_modown(),

 - which does

	write_lock_irq(&filp->f_owner.lock);
	..
	write_unlock_irq(&filp->f_owner.lock);

   which in turn enables interrupts while we still hold ctrl_lock.

so that whole commit 70362511806 was/is buggy in mainline too. 

The minimal fix is likely to just make f_modown() use 
write_lock_irqsave/restore. Greg?

		Linus
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