Messages in this thread Patch in this message | | | Subject | Re: futex question | From | Peter Zijlstra <> | Date | Mon, 05 Oct 2009 13:58:54 +0200 |
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On Mon, 2009-10-05 at 12:36 +0200, Peter Zijlstra wrote: > On Sun, 2009-10-04 at 18:59 +0200, Thomas Gleixner wrote: > > > > do. It does not feel right. Currently, with or without my change, > > > such a thing would indefinitely block other waiters on the same > > > futex. > > > > Right. Which completely defeats the purpose of the robust list. Will > > have a look tomorrow. > > Right, so mm_release() which is meant to destroy the old mm context > actually does exit_robust_list(), but the problem is that it does so on > the new mm, not the old one that got passed down to mm_release(). > > The other detail is that exit_robust_list() doesn't clear > current->robust_list. > > The problem with the patch send my Ani is that it clears the robust > lists before the point of no return, so on a failing execve() we'd have > messed up the state. > > Making exit_robust_list() deal with an mm that is not the current mm is > interesting indeed.
Hmm...
static int exec_mmap(struct mm_struct *mm) { struct task_struct *tsk; struct mm_struct * old_mm, *active_mm;
/* Notify parent that we're no longer interested in the old VM */ tsk = current; old_mm = current->mm; mm_release(tsk, old_mm);
if (old_mm) { /* * Make sure that if there is a core dump in progress * for the old mm, we get out and die instead of going * through with the exec. We must hold mmap_sem around * checking core_state and changing tsk->mm. */ down_read(&old_mm->mmap_sem); if (unlikely(old_mm->core_state)) { up_read(&old_mm->mmap_sem); return -EINTR; } } task_lock(tsk); active_mm = tsk->active_mm; tsk->mm = mm; tsk->active_mm = mm; activate_mm(active_mm, mm); task_unlock(tsk); arch_pick_mmap_layout(mm); if (old_mm) { up_read(&old_mm->mmap_sem); BUG_ON(active_mm != old_mm); mm_update_next_owner(old_mm); mmput(old_mm); return 0; } mmdrop(active_mm); return 0; }
Actually calls mm_release() before the flip, so the below might actually be sufficient?
--- kernel/fork.c | 10 ++++++++-- 1 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c index 266c6af..4c20fff 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -570,12 +570,18 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm) /* Get rid of any futexes when releasing the mm */ #ifdef CONFIG_FUTEX - if (unlikely(tsk->robust_list)) + if (unlikely(tsk->robust_list)) { exit_robust_list(tsk); + tsk->robust_list = NULL; + } #ifdef CONFIG_COMPAT - if (unlikely(tsk->compat_robust_list)) + if (unlikely(tsk->compat_robust_list)) { compat_exit_robust_list(tsk); + tsk->compat_robust_list = NULL; + } #endif + if (unlikely(!list_empty(&tsk->pi_state_list))) + exit_pi_state_list(tsk); #endif /* Get rid of any cached register state */
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