Messages in this thread | | | Date | Fri, 18 Aug 2023 17:26:34 +0800 | Subject | Re: [RFC PATCH -next] mm: fix softlockup by replacing tasklist_lock with RCU in for_each_process() | From | Tong Tiangen <> |
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在 2023/8/17 13:36, Naoya Horiguchi 写道: > 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@huawei.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
Thank you for your reply. Since tasklist_lock is no longer used in collect_procs_xxx(), Should I delete these two comments in mm/filemap.c?
Thanks, Tong.
> >> --- >> 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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