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Date: Mon, 29 Aug 2011 16:25:08 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: Hans Petter Selasky <hselasky@FreeBSD.org>
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Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org,
	src-committers@FreeBSD.org
Subject: Re: svn commit: r223989 - head/sys/dev/usb/input
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on 14/07/2011 00:07 Hans Petter Selasky said the following:
> Author: hselasky
> Date: Wed Jul 13 21:07:50 2011
> New Revision: 223989
> URL: http://svn.freebsd.org/changeset/base/223989
> 
> Log:
>   Fix for dump after shutdown with USB keyboard plugged in. It appears that the
>   system timer is stopped during shutdown and that the pause() statement in ukbd
>   causes infinite hang in this regard. The fix is to use mi_switch() instead of
>   pause() to do the required task switch to ensure that the required USB processes
>   get executed.
>   
>   Reported by:	Mike_Karels@mcafee.com
>   MFC after:	1 week
> 
> Modified:
>   head/sys/dev/usb/input/ukbd.c
> 
> Modified: head/sys/dev/usb/input/ukbd.c
> ==============================================================================
> --- head/sys/dev/usb/input/ukbd.c	Wed Jul 13 21:07:41 2011	(r223988)
> +++ head/sys/dev/usb/input/ukbd.c	Wed Jul 13 21:07:50 2011	(r223989)
> @@ -59,6 +59,8 @@ __FBSDID("$FreeBSD$");
>  #include <sys/callout.h>
>  #include <sys/malloc.h>
>  #include <sys/priv.h>
> +#include <sys/proc.h>
> +#include <sys/sched.h>
>  #include <sys/kdb.h>
>  
>  #include <dev/usb/usb.h>
> @@ -386,6 +388,33 @@ ukbd_put_key(struct ukbd_softc *sc, uint
>  }
>  
>  static void
> +ukbd_yield(void)
> +{
> +	struct thread *td = curthread;
> +	uint32_t old_prio;
> +
> +	DROP_GIANT();
> +
> +	thread_lock(td);
> +
> +	/* get current priority */
> +	old_prio = td->td_base_pri;
> +
> +	/* set new priority */
> +	sched_prio(td, td->td_user_pri);
> +
> +	/* cause a task switch */
> +	mi_switch(SW_INVOL | SWT_RELINQUISH, NULL);
> +
> +	/* restore priority */
> +	sched_prio(td, old_prio);
> +
> +	thread_unlock(td);
> +
> +	PICKUP_GIANT();
> +}
> +

Another question - why ukbd_yield() is needed?
Why kern_yield() would not be good here?
What are the priority manipulations in ukbd_yield?
Not saying that the code is incorrect, just that this is not explained in the
commit message.

-- 
Andriy Gapon