linux-kernel - Re: [PATCH] i386 TIF flags for debug regs and io bitmap in ctxsw (v2)

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Date:	Tue, 25 Jul 2006 15:19:49 -0400
From:	Chuck Ebbert <76306.1226@...puserve.com>
To:	Stephane Eranian <eranian@....hp.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] i386 TIF flags for debug regs and io bitmap in
  ctxsw (v2)

In-Reply-To: <20060725055439.GA18053@...nkl.hpl.hp.com>

On Mon, 24 Jul 2006 22:54:39 -0700, Stephane Eranian wrote:
>
> I would like to follow-up the TIF flags and especially on the
> rules of inheritance. It appears the TIF flags are copied from
> parent to child task systematically on copy_process.
> Then they are adjusted in copy_threads() or sub-functions.
> 
> The TIF_IO_BITMAP is checked in copy_threads() with the following code:
> 
> int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
>       unsigned long unused,
>       struct task_struct * p, struct pt_regs * regs)
> {
>       ....
>       if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
>               p->thread.io_bitmap_ptr = kmalloc(IO_BITMAP_BYTES, GFP_KERNEL);
>               if (!p->thread.io_bitmap_ptr) {
>                       p->thread.io_bitmap_max = 0;
>                       return -ENOMEM;
>               }
>               memcpy(p->thread.io_bitmap_ptr, tsk->thread.io_bitmap_ptr,
>                       IO_BITMAP_BYTES);
>               set_tsk_thread_flag(p, TIF_IO_BITMAP);
>       }
> 
> I would think that the set_tsk_thread_flag() is extraneous.
> 

Yeah.  But harmless.

> As for TIF_DEBUG, my patch is not clearing it. I don't think you can
> have HW breakpoints be inherited from one task to the other.

Looks like the debug regs get copied on fork and only cleared on exec
in flush_thread().  So this should be OK.  Please doublecheck.

(The new TIF_DEBUG flag went into 2.8.18-rc, in case you didn't notice.)

-- 
Chuck

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