2.6.xxx race condition in x86_64's global_flush_tlb???

2.6.xxx race condition in x86_64's global_flush_tlb???

[Doug Reiland]

I have seen some hangs in 2.6-x86_64 in flush_kernel_map(). The tests
cause alot of ioremap/iounmap to occur concurrently across many
processor threads.

Looking at the hung processor hangs, they are looping in
flush_kernel_map() and the list they get from the smp_call_function()
appears to be corrupt. In fact, I see deferred_pages as an entry and
that isn't supposed to happen.

I am questioning the locking in global_flush_tlb() listed below. The
down_read/up_read protection doesn't seen safe. If several threads are
rushing thru here, deferred_pages could be getting changed as they
look at it. I don't think there any protection when
list_replace_init() calls INIT_LIST_HEAD().

I changed the down_read()/up_read() around list_replace_init() to
down_write()/up_write() and my test runs fine.


void global_flush_tlb(void)
{
        struct page *pg, *next;
        struct list_head l;

        down_read(&init_mm.mmap_sem); // XXX should be down_write()???
        list_replace_init(&deferred_pages, &l);
        up_read(&init_mm.mmap_sem); // XXX should be up_write()????
        flush_map(&l);

        list_for_each_entry_safe(pg, next, &l, lru) {
                ClearPagePrivate(pg);
                __free_page(pg);
        }
}

Re: 2.6.xxx race condition in x86_64's global_flush_tlb???

[Randy Dunlap]

On Wed, 24 Oct 2007 16:39:57 -0400 Doug Reiland wrote:

> I have seen some hangs in 2.6-x86_64 in flush_kernel_map(). The tests
> cause alot of ioremap/iounmap to occur concurrently across many
> processor threads.
> 
> Looking at the hung processor hangs, they are looping in
> flush_kernel_map() and the list they get from the smp_call_function()
> appears to be corrupt. In fact, I see deferred_pages as an entry and
> that isn't supposed to happen.
> 
> I am questioning the locking in global_flush_tlb() listed below. The
> down_read/up_read protection doesn't seen safe. If several threads are
> rushing thru here, deferred_pages could be getting changed as they
> look at it. I don't think there any protection when
> list_replace_init() calls INIT_LIST_HEAD().
> 
> I changed the down_read()/up_read() around list_replace_init() to
> down_write()/up_write() and my test runs fine.
> 
> 
> void global_flush_tlb(void)
> {
>         struct page *pg, *next;
>         struct list_head l;
> 
>         down_read(&init_mm.mmap_sem); // XXX should be down_write()???
>         list_replace_init(&deferred_pages, &l);
>         up_read(&init_mm.mmap_sem); // XXX should be up_write()????
>         flush_map(&l);
> 
>         list_for_each_entry_safe(pg, next, &l, lru) {
>                 ClearPagePrivate(pg);
>                 __free_page(pg);
>         }
> }

Seems to be already fixed in current git tree.

---
~Randy

Re: 2.6.xxx race condition in x86_64's global_flush_tlb???

[Doug Reiland]

Your right. I thought I was updated but was at 23-rc9.
Sorry!

On 10/24/07, Randy Dunlap <rdunlap@xenotime.net> wrote:
> On Wed, 24 Oct 2007 16:39:57 -0400 Doug Reiland wrote:
>
> > I have seen some hangs in 2.6-x86_64 in flush_kernel_map(). The tests
> > cause alot of ioremap/iounmap to occur concurrently across many
> > processor threads.
> >
> > Looking at the hung processor hangs, they are looping in
> > flush_kernel_map() and the list they get from the smp_call_function()
> > appears to be corrupt. In fact, I see deferred_pages as an entry and
> > that isn't supposed to happen.
> >
> > I am questioning the locking in global_flush_tlb() listed below. The
> > down_read/up_read protection doesn't seen safe. If several threads are
> > rushing thru here, deferred_pages could be getting changed as they
> > look at it. I don't think there any protection when
> > list_replace_init() calls INIT_LIST_HEAD().
> >
> > I changed the down_read()/up_read() around list_replace_init() to
> > down_write()/up_write() and my test runs fine.
> >
> >
> > void global_flush_tlb(void)
> > {
> >         struct page *pg, *next;
> >         struct list_head l;
> >
> >         down_read(&init_mm.mmap_sem); // XXX should be down_write()???
> >         list_replace_init(&deferred_pages, &l);
> >         up_read(&init_mm.mmap_sem); // XXX should be up_write()????
> >         flush_map(&l);
> >
> >         list_for_each_entry_safe(pg, next, &l, lru) {
> >                 ClearPagePrivate(pg);
> >                 __free_page(pg);
> >         }
> > }
>
> Seems to be already fixed in current git tree.
>
> ---
> ~Randy
>