On Wed 21-06-23 12:40:34, Sebastian Andrzej Siewior wrote:
> __build_all_zonelists() acquires zonelist_update_seq by first disabling
> interrupts via local_irq_save() and then acquiring the seqlock with
> write_seqlock(). This is troublesome and leads to problems on
> PREEMPT_RT because the inner spinlock_t is now acquired with disabled
> interrupts.
And the spinlock might sleep with PREEMPT_RT so a deadlock, right? It
would be better to call that out explicitly
> The API provides write_seqlock_irqsave() which does the right thing in
> one step.
> printk_deferred_enter() has to be invoked in non-migrate-able context to
> ensure that deferred printing is enabled and disabled on the same CPU.
> This is the case after zonelist_update_seq has been acquired.
>
> Acquire zonelist_update_seq with write_seqlock_irqsave() and then defer
> printk output.
>
> Fixes: 1007843a91909 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks
> ---
> mm/page_alloc.c | 11 ++++-------
> 1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47421bedc12b7..99b7e7d09c5c0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5808,11 +5808,10 @@ static void __build_all_zonelists(void *data)
> unsigned long flags;
>
> /*
> - * Explicitly disable this CPU's interrupts before taking seqlock
> - * to prevent any IRQ handler from calling into the page allocator
> - * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
> + * The zonelist_update_seq must be acquired with irqsave because the
> + * reader can be invoked from IRQ with GFP_ATOMIC.
> */
> - local_irq_save(flags);
> + write_seqlock_irqsave(&zonelist_update_seq, flags);
> /*
> * Explicitly disable this CPU's synchronous printk() before taking
> * seqlock to prevent any printk() from trying to hold port->lock, for
> @@ -5820,7 +5819,6 @@ static void __build_all_zonelists(void *data)
> * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
> */
> printk_deferred_enter();
> - write_seqlock(&zonelist_update_seq);
>
> #ifdef CONFIG_NUMA
> memset(node_load, 0, sizeof(node_load));
> @@ -5857,9 +5855,8 @@ static void __build_all_zonelists(void *data)
> #endif
> }
>
> - write_sequnlock(&zonelist_update_seq);
> printk_deferred_exit();
> - local_irq_restore(flags);
> + write_sequnlock_irqrestore(&zonelist_update_seq, flags);
> }
>
> static noinline void __init
> --
> 2.40.1
--
Michal Hocko
SUSE Labs
