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>
---
Changes since RFC[1]:
- 1. According to Naoya's suggestion, modify the tasklist_lock in the
comment about locking order in mm/filemap.c.
- 2. According to Kefeng's suggestion, optimize the implementation of
find_early_kill_thread() without functional changes.
- 3. Modify the title description.
[1] https://lore.kernel.org/lkml/20230815130154.1100779-1-tongtiangen@xxxxxxxxxx/
---
mm/filemap.c | 3 ---
mm/ksm.c | 4 ++--
mm/memory-failure.c | 26 ++++++++++++++------------
3 files changed, 16 insertions(+), 17 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index 014b73eb96a1..dfade1ef1765 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -121,9 +121,6 @@
* bdi.wb->list_lock (zap_pte_range->set_page_dirty)
* ->inode->i_lock (zap_pte_range->set_page_dirty)
* ->private_lock (zap_pte_range->block_dirty_folio)
- *
- * ->i_mmap_rwsem
- * ->tasklist_lock (memory_failure, collect_procs_ao)
*/
static void page_cache_delete(struct address_space *mapping,
diff --git a/mm/ksm.c b/mm/ksm.c
index 8d6aee05421d..981af9c72e7a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2925,7 +2925,7 @@ 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;
@@ -2944,7 +2944,7 @@ 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..3e4fd8beec31 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -546,24 +546,26 @@ 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;
+ rcu_read_lock();
for_each_thread(tsk, t) {
if (t->flags & PF_MCE_PROCESS) {
if (t->flags & PF_MCE_EARLY)
- return t;
+ goto found;
} else {
if (sysctl_memory_failure_early_kill)
- return t;
+ goto found;
}
}
- return NULL;
+
+ t = NULL;
+found:
+ rcu_read_unlock();
+ return t;
}
/*
@@ -609,7 +611,7 @@ 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 +628,7 @@ 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 +644,7 @@ 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 +664,7 @@ 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 +687,7 @@ 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 +698,7 @@ 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
