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uprobes: Fix the waitqueue_active() check in xol_free_insn_slot()
The xol_free_insn_slot()->waitqueue_active() check is buggy. We need mb() after we set the conditon for wait_event(), or xol_take_insn_slot() can miss the wakeup. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Pratyush Anand <panand@redhat.com> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20150721134036.GA4799@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -1337,6 +1337,7 @@ static void xol_free_insn_slot(struct task_struct *tsk)
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clear_bit(slot_nr, area->bitmap);
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atomic_dec(&area->slot_count);
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smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
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if (waitqueue_active(&area->wq))
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wake_up(&area->wq);
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