linux-kernel - Re: Thread flags modified without set_thread

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Date:	Thu, 01 Mar 2007 11:59:07 -0800
From:	Mathieu Desnoyers <compudj@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Haavard Skinnemoen <hskinnemoen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Thread flags modified without set_thread_flag() (non atomically)

Andrew Morton wrote:
> On Thu, 1 Mar 2007 10:34:51 +0100 Haavard Skinnemoen <hskinnemoen@...el.com> wrote:
>
>   
>>> On Mon, 26 Feb 2007 12:10:37 -0800 Mathieu Desnoyers <compudj@...gle.com> wrote:
>>>       
>>>> 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.
>>>
>>>
>>>       
>>>> avr32/kernel/ptrace.c:                  ti->flags |= TIF_SINGLE_STEP;
>>>>         
>>> heh.  Haarvard, you got a bug.
>>>       
>> Heh, yeah. That would indeed explain some strange gdb behaviour. It
>> will only trigger when single-stepping into an exception or interrupt
>> handler so thanks for pointing it out; I would have had a hard time
>> figuring it out on my own...
>>     
>
> yup, tricky.
>
> If there's a lesson here, it is "don't provide #defines in the header for
> both versions".
>
>   

Hrm, but the bitmask version is useful (and correctly used) whenever
the flag is read and tested.

The proper way to do this would be to change every use the _TIF_* flag 
in a flag comparison
for a call to test_ti_thread_flag(). I wonder if gcc optimizes multiple 
constant test_bit() applying
on the same variable linked by logical and/or so it becomes a single 
read and a small set of comparisons.

> The block code does a similar thing:
>
> #define REQ_RW          (1 << __REQ_RW)
> #define REQ_FAILFAST    (1 << __REQ_FAILFAST)
> #define REQ_SORTED      (1 << __REQ_SORTED)
> #define REQ_SOFTBARRIER (1 << __REQ_SOFTBARRIER)
>
> and I've caught Jens using the wrong identifier at least twice in the past.
>
> It's better I think to just provide #defines for the bit offsets and
> open-code the shifting if needed.  Like PG_foo and BH_Foo.
>
>   
>> I don't think either of those need to be atomic though, since both of
>> them happen in monitor mode with interrupts disabled.
>>     
>
> That's true until you implement SMP ;)
>   

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