RE: 2.6.9 chrdev_open: serial_core: uart_open

["karl malbrain" <karl@petzent.com>]

> -----Original Message-----
> From: Russell King
> Sent: Friday, July 15, 2005 12:23 AM
> To: karl malbrain
> Cc: Linux-Kernel@Vger. Kernel. Org
> Subject: Re: 2.6.9 chrdev_open: serial_core: uart_open
>
>
> On Thu, Jul 14, 2005 at 04:50:00PM -0700, karl malbrain wrote:
> > chrdev_open issues a lock_kernel() before calling uart_open.
> >
> > It would appear that servicing the blocking open request
> uart_open goes to
> > sleep with the kernel locked.  Would this shut down subsequent access to
> > opening "/dev/tty"???
>
> No.  lock_kernel() is automatically released when a process sleeps.

Drilling down between the uart_open and chrdev_open into tty_open is a
semaphore tty_sem that is being held during the sleep cycle in uart_open.

This would appear to be the problem!!  Is this a new semaphore in 2.6? How
could this have ever worked with tty blocking mode?  It would appear that
tty_sem is going to have to be released before sleeping in uart_open.  What
a mess.

N.b. I don't pretend to understand how uart_change_pm, uart_startup, and
uart_block_til_ready could ALL be on the call stack.  Uart_open calls them
sequentially.  Perhaps you might explain how this works?  Thanks, karl m



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