Re: [RFC PATCH -next] mm: fix softlockup by replacing tasklist_lock with RCU in for_each_process()

[Naoya Horiguchi]

On Tue, Aug 15, 2023 at 09:01:54PM +0800, Tong Tiangen wrote:
> We found a softlock issue in our test, analyzed the logs, and found that
> the relevant CPU call trace as follows:
> 
> CPU0:
>   _do_fork
>     -> copy_process()
>       -> write_lock_irq(&tasklist_lock)  //Disable irq,waiting for
>       					 //tasklist_lock
> 
> CPU1:
>   wp_page_copy()
>     ->pte_offset_map_lock()
>       -> spin_lock(&page->ptl);        //Hold page->ptl
>     -> ptep_clear_flush()
>       -> flush_tlb_others() ...
>         -> smp_call_function_many()
>           -> arch_send_call_function_ipi_mask()
>             -> csd_lock_wait()         //Waiting for other CPUs respond
> 	                               //IPI
> 
> CPU2:
>   collect_procs_anon()
>     -> read_lock(&tasklist_lock)       //Hold tasklist_lock
>       ->for_each_process(tsk)
>         -> page_mapped_in_vma()
>           -> page_vma_mapped_walk()
> 	    -> map_pte()
>               ->spin_lock(&page->ptl)  //Waiting for page->ptl
> 
> We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2
> unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result,
> softlockup is triggered.
> 
> For collect_procs_anon(), we will not modify the tasklist, but only perform
> read traversal. Therefore, we can use rcu lock instead of spin lock
> tasklist_lock, from this, we can break the softlock chain above.
> 
> The same logic can also be applied to:
>  - collect_procs_file()
>  - collect_procs_fsdax()
>  - collect_procs_ksm()
>  - find_early_kill_thread()
> 
> Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>

Hello Tiangen, thank you for finding the issue.
mm/filemap.c mentions tasklist_lock in the comment about locking order,

  * ->i_mmap_rwsem                                                      
  *   ->tasklist_lock            (memory_failure, collect_procs_ao)     

so you can update this together?
Otherwise looks good to me.

Thanks,
Naoya Horiguchi

> ---
>  mm/ksm.c            |  4 ++--
>  mm/memory-failure.c | 36 ++++++++++++++++++++++--------------
>  2 files changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 6b7b8928fb96..dcbc0c7f68e7 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2919,7 +2919,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
>  		struct anon_vma *av = rmap_item->anon_vma;
>  
>  		anon_vma_lock_read(av);
> -		read_lock(&tasklist_lock);
> +		rcu_read_lock();
>  		for_each_process(tsk) {
>  			struct anon_vma_chain *vmac;
>  			unsigned long addr;
> @@ -2938,7 +2938,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
>  				}
>  			}
>  		}
> -		read_unlock(&tasklist_lock);
> +		rcu_read_unlock();
>  		anon_vma_unlock_read(av);
>  	}
>  }
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7b01fffe7a79..6a02706043f4 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -546,24 +546,32 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>   * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
>   * on behalf of the thread group. Return task_struct of the (first found)
>   * dedicated thread if found, and return NULL otherwise.
> - *
> - * We already hold read_lock(&tasklist_lock) in the caller, so we don't
> - * have to call rcu_read_lock/unlock() in this function.
>   */
>  static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
>  {
>  	struct task_struct *t;
> +	bool find = false;
>  
> +	rcu_read_lock();
>  	for_each_thread(tsk, t) {
>  		if (t->flags & PF_MCE_PROCESS) {
> -			if (t->flags & PF_MCE_EARLY)
> -				return t;
> +			if (t->flags & PF_MCE_EARLY) {
> +				find = true;
> +				break;
> +			}
>  		} else {
> -			if (sysctl_memory_failure_early_kill)
> -				return t;
> +			if (sysctl_memory_failure_early_kill) {
> +				find = true;
> +				break;
> +			}
>  		}
>  	}
> -	return NULL;
> +	rcu_read_unlock();
> +
> +	if (!find)
> +		t = NULL;
> +
> +	return t;
>  }
>  
>  /*
> @@ -609,7 +617,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
>  		return;
>  
>  	pgoff = page_to_pgoff(page);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	for_each_process(tsk) {
>  		struct anon_vma_chain *vmac;
>  		struct task_struct *t = task_early_kill(tsk, force_early);
> @@ -626,7 +634,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
>  			add_to_kill_anon_file(t, page, vma, to_kill);
>  		}
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  	anon_vma_unlock_read(av);
>  }
>  
> @@ -642,7 +650,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
>  	pgoff_t pgoff;
>  
>  	i_mmap_lock_read(mapping);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	pgoff = page_to_pgoff(page);
>  	for_each_process(tsk) {
>  		struct task_struct *t = task_early_kill(tsk, force_early);
> @@ -662,7 +670,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
>  				add_to_kill_anon_file(t, page, vma, to_kill);
>  		}
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  	i_mmap_unlock_read(mapping);
>  }
>  
> @@ -685,7 +693,7 @@ static void collect_procs_fsdax(struct page *page,
>  	struct task_struct *tsk;
>  
>  	i_mmap_lock_read(mapping);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	for_each_process(tsk) {
>  		struct task_struct *t = task_early_kill(tsk, true);
>  
> @@ -696,7 +704,7 @@ static void collect_procs_fsdax(struct page *page,
>  				add_to_kill_fsdax(t, page, vma, to_kill, pgoff);
>  		}
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  	i_mmap_unlock_read(mapping);
>  }
>  #endif /* CONFIG_FS_DAX */
> -- 
> 2.25.1
> 
> 
>