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Date: Thu, 17 Aug 2023 14:36:30 +0900
From: Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To: Tong Tiangen <tongtiangen@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Naoya Horiguchi <naoya.horiguchi@....com>,
Miaohe Lin <linmiaohe@...wei.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, wangkefeng.wang@...wei.com,
Guohanjun <guohanjun@...wei.com>
Subject: Re: [RFC PATCH -next] mm: fix softlockup by replacing tasklist_lock
with RCU in for_each_process()
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@...wei.com>
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
>
>
>
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