On Tue, Dec 13, 2016 at 11:21 AM, Andrey Ryabinin <[email protected]> wrote: > On 12/13/2016 09:15 PM, Andy Lutomirski wrote: > > >>>> >>>> But not quite acked by me. What happened to the vfree code that >>>> causes vfree_deferred to be called in a preemptable context? That >>>> sounds like a bug. >>> >>> Not sure I understand but the above stack points to a preemptible >>> context (copy_process). My stack was different and it looks preemptible as >>> well. >>> free_thread_stack calls vfree_atomic unconditionally. So I am not sure >>> why do you think this is a bug? >>> >>>> (This code doesn't exist in Linus' tree. What tree does this apply to.) >>> >>> Anyway, now that I am looking at Andrew's tree I can see [1] which >>> doesn't have this_cpu_ptr. So I am not sure where this this_cpu_ptr came >>> from. Maybe the previous version of the patch which has shown up in the >>> linux-next and Andrew has picked up [2] in the meantime. /me confused >>> >>> [1] http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-add-vfree_atomic.patch >>> [2] >>> http://lkml.kernel.org/r/[email protected] >> >> The underlying issue seems to be that we have this shiny new function >> vfree_atomic() which doesn't work in *non-atomic* context and that we > > It does work non-atomic context. It's fixed now. > >> have "kernel/fork: use vfree_atomic() to free thread stack" that calls >> vfree_atomic() from non-atomic context. > > From both context actually. Usually task stack is freed from atomic context: > > http://lkml.kernel.org/r/[email protected] > > http://lkml.kernel.org/r/calcetrvqjejgpqvudem8rk3uxdegfozy4cojqjqjcltbdnj...@mail.gmail.com > > On rare occasions it can be freed from non-atomic context, e.g. error path in > copy_process(). > >> I'm not sure what the motivation of the latter patch was, but ISTM we >> should revert it. TBH I'm not quite sure what the purpose of >> splitting vfree() and vfree_atomic() was, but I'm not seeing any >> reason that the common case of freeing stacks from non-atomic context >> should defer the free instead of just doing it right away. >> >> Andrey, Johannes, why should task stack freeing use vfree_atomic() in >> the first place? > > Because vfree() now can sleep and task stack freeing usually done in atomic > context. > >
Fair enough. -- Andy Lutomirski AMA Capital Management, LLC
