Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

Michal Hocko
2019-10-09T14:34:43Z

On Wed 09-10-19 10:19:44, Qian Cai wrote:
> On Wed, 2019-10-09 at 15:51 +0200, Michal Hocko wrote:
[...]
> > Can you paste the full lock chain graph to be sure we are on the same
> > page?
>
> WARNING: possible circular locking dependency detected
> 5.3.0-next-20190917 #8 Not tainted
> ------------------------------------------------------
> test.sh/8653 is trying to acquire lock:
> ffffffff865a4460 (console_owner){-.-.}, at:
> console_unlock+0x207/0x750
>
> but task is already holding lock:
> ffff88883fff3c58 (&(&zone->lock)->rlock){-.-.}, at:
> __offline_isolated_pages+0x179/0x3e0
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #3 (&(&zone->lock)->rlock){-.-.}:
>        __lock_acquire+0x5b3/0xb40
>        lock_acquire+0x126/0x280
>        _raw_spin_lock+0x2f/0x40
>        rmqueue_bulk.constprop.21+0xb6/0x1160
>        get_page_from_freelist+0x898/0x22c0
>        __alloc_pages_nodemask+0x2f3/0x1cd0
>        alloc_pages_current+0x9c/0x110
>        allocate_slab+0x4c6/0x19c0
>        new_slab+0x46/0x70
>        ___slab_alloc+0x58b/0x960
>        __slab_alloc+0x43/0x70
>        __kmalloc+0x3ad/0x4b0
>        __tty_buffer_request_room+0x100/0x250
>        tty_insert_flip_string_fixed_flag+0x67/0x110
>        pty_write+0xa2/0xf0
>        n_tty_write+0x36b/0x7b0
>        tty_write+0x284/0x4c0
>        __vfs_write+0x50/0xa0
>        vfs_write+0x105/0x290
>        redirected_tty_write+0x6a/0xc0
>        do_iter_write+0x248/0x2a0
>        vfs_writev+0x106/0x1e0
>        do_writev+0xd4/0x180
>        __x64_sys_writev+0x45/0x50
>        do_syscall_64+0xcc/0x76c
>        entry_SYSCALL_64_after_hwframe+0x49/0xbe

This one looks indeed legit. pty_write is allocating memory from inside
the port->lock. But this seems to be quite broken, right? The forward
progress depends on GFP_ATOMIC allocation which might fail easily under
memory pressure. So the preferred way to fix this should be to change
the allocation scheme to use the preallocated buffer and size it from a
context when it doesn't hold internal locks. It might be a more complex
fix than using printk_deferred or other games but addressing that would
make the pty code more robust as well.
--
Michal Hocko
SUSE Labs