linux-kernel - Re: Thread flags modified without set_thread

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Message-Id: <20070228.222341.74751530.davem@davemloft.net>
Date:	Wed, 28 Feb 2007 22:23:41 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	akpm@...ux-foundation.org
Cc:	compudj@...gle.com, mbligh@...gle.com,
	linux-kernel@...r.kernel.org, ak@...e.de, paulus@...ba.org,
	tony.luck@...el.com, hskinnemoen@...el.com
Subject: Re: Thread flags modified without set_thread_flag() (non
 atomically)

From: Andrew Morton <akpm@...ux-foundation.org>
Date: Wed, 28 Feb 2007 22:03:49 -0800

> On Mon, 26 Feb 2007 12:10:37 -0800 Mathieu Desnoyers <compudj@...gle.com> wrote:
> 
> > Other examples :
> > 
> > sparc64/kernel/ptrace.c:                if 
> > ((task_thread_info(child)->flags & _TIF_32BIT) != 0) {
> > sparc64/kernel/process.c:               t->flags ^= (_TIF_ABI_PENDING | 
> > _TIF_32BIT);
> > sparc64/kernel/process.c:                       t->flags &= ~_TIF_PERFCTR;
> > 
> > sparc/kernel/process.c:         current_thread_info()->flags &= 
> > ~_TIF_USEDFPU;
> > sparc/kernel/process.c:         current_thread_info()->flags &= 
> > ~_TIF_USEDFPU;
> > sparc/kernel/process.c:         current_thread_info()->flags &= 
> > ~_TIF_USEDFPU;
> > sparc/kernel/process.c:                 current_thread_info()->flags &= 
> > ~(_TIF_USEDFPU);
> > sparc/kernel/traps.c:   current_thread_info()->flags |= _TIF_USEDFPU;
> > sparc/kernel/traps.c:   task_thread_info(fpt)->flags &= ~_TIF_USEDFPU;
> 
> That all looks rather deliberate.
> 
> > powerpc/kernel/process.c:               t->flags ^= (_TIF_ABI_PENDING | 
> > _TIF_32BIT);
> >
> > ia64/kernel/mca.c:      ti->flags = _TIF_MCA_INIT;
> >
> > avr32/kernel/ptrace.c:          ti->flags |= _TIF_BREAKPOINT;
> 
> No, I don't immediately see anything in the flush_old_exec() code path
> which tells us that nobody else can look up this thread_info (or be holding
> a ref to it) in this context.

Provide the counter example, what other threads of control can modify
relevant flags while a thread is exec()'ing?  It's essentially frozen
outside of these code paths, otherwise we wouldn't be able to make all
of these modifications to the task state.

I think these cases are very safe.
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