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    SubjectRe: current_thread_info() vs task_thread_info(current)
    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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