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    Subject[PATCH 4.4 227/312] sched/preempt: Fix preempt_count manipulations
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    commit 2e636d5e66c35dfcbaf617aa8fa963f6847478fe upstream.

    Vikram reported that his ARM64 compiler managed to 'optimize' away the
    preempt_count manipulations in code like:

    preempt_enable_no_resched();
    put_user();
    preempt_disable();

    Irrespective of that fact that that is horrible code that should be
    fixed for many reasons, it does highlight a deficiency in the generic
    preempt_count manipulators. As it is never right to combine/elide
    preempt_count manipulations like this.

    Therefore sprinkle some volatile in the two generic accessors to
    ensure the compiler is aware of the fact that the preempt_count is
    observed outside of the regular program-order view and thus cannot be
    optimized away like this.

    x86; the only arch not using the generic code is not affected as we
    do all this in asm in order to use the segment base per-cpu stuff.

    Reported-by: Vikram Mulukutla <markivx@codeaurora.org>
    Tested-by: Vikram Mulukutla <markivx@codeaurora.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Fixes: a787870924db ("sched, arch: Create asm/preempt.h")
    Link: http://lkml.kernel.org/r/20160516131751.GH3205@twins.programming.kicks-ass.net
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/asm-generic/preempt.h | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/include/asm-generic/preempt.h
    +++ b/include/asm-generic/preempt.h
    @@ -7,10 +7,10 @@

    static __always_inline int preempt_count(void)
    {
    - return current_thread_info()->preempt_count;
    + return READ_ONCE(current_thread_info()->preempt_count);
    }

    -static __always_inline int *preempt_count_ptr(void)
    +static __always_inline volatile int *preempt_count_ptr(void)
    {
    return &current_thread_info()->preempt_count;
    }

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