[PATCH 2/5] HWPOISON: fix tasklist_lock/anon_vma locking order

[Wu Fengguang <fengguang.wu@intel.com>]

[-- Attachment #1: hwpoison-lock-order.patch --]
[-- Type: text/plain, Size: 1556 bytes --]

To avoid possible deadlock. Proposed by Nick Piggin:

  You have tasklist_lock(R) nesting outside i_mmap_lock, and inside anon_vma
  lock. And anon_vma lock nests inside i_mmap_lock.

  This seems fragile. If rwlocks ever become FIFO or tasklist_lock changes
  type (maybe -rt kernels do it), then you could have a task holding
  anon_vma lock and waiting for tasklist_lock, and another holding tasklist
  lock and waiting for i_mmap_lock, and another holding i_mmap_lock and
  waiting for anon_vma lock.

CC: Nick Piggin <npiggin@suse.de>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/memory-failure.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- sound-2.6.orig/mm/memory-failure.c
+++ sound-2.6/mm/memory-failure.c
@@ -215,12 +215,14 @@ static void collect_procs_anon(struct pa
 {
 	struct vm_area_struct *vma;
 	struct task_struct *tsk;
-	struct anon_vma *av = page_lock_anon_vma(page);
+	struct anon_vma *av;
 
+	read_lock(&tasklist_lock);
+
+	av = page_lock_anon_vma(page);
 	if (av == NULL) /* Not actually mapped anymore */
-		return;
+		goto out;
 
-	read_lock(&tasklist_lock);
 	for_each_process (tsk) {
 		if (!tsk->mm)
 			continue;
@@ -230,6 +232,7 @@ static void collect_procs_anon(struct pa
 		}
 	}
 	page_unlock_anon_vma(av);
+out:
 	read_unlock(&tasklist_lock);
 }
 

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