PATCH? exit_mm: update the comments, move atomic_inc(mm_count) outside of mmap_sem
From: Oleg Nesterov
Date: Wed Jul 16 2008 - 13:12:35 EST
(textually depends on
"[PATCH 2/2] coredump: exit_mm: clear ->mm first, then play with ->core_state")
More the "is my understanding correct" than a patch. Assuming it is correct:
- explain the subtle atomic_inc(&mm->mm_count).
- move it outside of mm->mmap_sem, otherwise it is not clear why
do we need this lock to bump ->mm_count.
- "fix" the comment above task_lock(). This lock is really needed
for get_task_mm/etc. Yes, we can change the code to use smp_mb()
+ spin_unlock_wait() but I guess we also have another reason.
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
--- 26-rc2/kernel/exit.c~EXIT_MM 2008-07-15 20:24:50.000000000 +0400
+++ 26-rc2/kernel/exit.c 2008-07-16 20:59:34.000000000 +0400
@@ -680,6 +680,14 @@ static void exit_mm(struct task_struct *
+ * We are going to clear ->mm. The next schedule() after that
+ * will assume we did context_switch()->atomic_inc(mm_count).
+ * finish_task_switch()->mmdrop() will "fix" the counter.
+ BUG_ON(mm != tsk->active_mm);
* Serialize with any possible pending coredump.
* We must hold mmap_sem around checking core_state
@@ -689,9 +697,7 @@ static void exit_mm(struct task_struct *
core_state = mm->core_state;
- BUG_ON(mm != tsk->active_mm);
- /* more a memory barrier than a real lock */
+ /* disables preemption, we must not schedule before enter_lazy_tlb() */
tsk->mm = NULL;
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