Re: current_thread_info() vs task_thread_info(current)

[Gleb Natapov]

On Mon, Jul 18, 2011 at 01:23:03PM +0200, Peter Zijlstra wrote:
> Thomas just spend a lovely morning trying to make sense of a trace where
> TIF_NEED_RESCHED wasn't set after resched_task() and magically appeared
> after local_bh_enable().
> 
> What happened is that on that particular platform softirqs ran on a
> separate stack, and current_thread_info() is relative to the stack
> pointer.
> 
> The result is that current_thread_info() isn't the same as
> task_thread_info(current), *surprise*!!
> 
> The immediate problem is of course that we can loose TIF flags when set
> through current_thread_info() from IRQ/SoftIRQ context.
> 
> Now I was going to add a WARN() in x86_64's current_thread_info() to
> catch all these, sadly x86_64's implementation isn't prone to this
> particular issue, which means most people (kernel devs) will not be
> affected (i386 is affected, but nobody sane uses that anymore).
> 
> 
> Just to give an example, RCU uses set_need_resched(), set_need_resched()
> uses current_thread_info(). The use in force_quiescent_state() is from
> softirq afaict, the one in __rcu_pending() is from hardirq.
> 
> On such platforms as Thomas was playing on, the TIF bit will be lost,
> since it will be set on the thread_info associated with some interrupt
> stack, not the current process.
> 
> 
> So how are we going to solve this? Naively I'd think that
> current_thread_info() is short for task_thread_info(current), and thus
> the platforms for where this isn't true are broken.
> 
> I mean, what use is the thread_info not of a thread?
> 
> Comments?

Why not use per cpu kernel_stack variable on all arches as x86_64 does?
How big the advantage of using stack pointer to find current thread info is?

--
			Gleb.
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