Re: [PATCH 5/9] blackfin: A couple of task-mm handling fixes

[Mike Frysinger]

On Monday 23 April 2012 03:09:01 Anton Vorontsov wrote:
 1. Working with task-mm w/o getting mm or grabing the task lock is
dangerous as -mm might disappear (exit_mm() assigns NULL under
task_lock(), so tasklist lock is not enough).

that isn't a problem for this code as it specifically checks if it's in an 
atomic section.  if it is, then task-mm can't go away on us.

We can't use get_task_mm()/mmput() pair as mmput() might sleep,
so we have to take the task lock while handle its mm.

if we're not in an atomic section, then sleeping is fine.

 2. Checking for process-mm is not enough because process' main
thread may exit or detach its mm via use_mm(), but other threads
may still have a valid mm.

i don't think it matters for this code (per the reasons above).

To catch this we use find_lock_task_mm(), which walks up all
threads and returns an appropriate task (with task lock held).

certainly fine for the non-atomic code path.  i guess we'll notice in crashes 
if it causes a problem in atomic code paths as well.
-mike


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[PATCH 5/9] blackfin: A couple of task-mm handling fixes

[Anton Vorontsov]

The patch fixes two problems:

1. Working with task-mm w/o getting mm or grabing the task lock is
   dangerous as -mm might disappear (exit_mm() assigns NULL under
   task_lock(), so tasklist lock is not enough).

   We can't use get_task_mm()/mmput() pair as mmput() might sleep,
   so we have to take the task lock while handle its mm.

2. Checking for process-mm is not enough because process' main
   thread may exit or detach its mm via use_mm(), but other threads
   may still have a valid mm.

   To catch this we use find_lock_task_mm(), which walks up all
   threads and returns an appropriate task (with task lock held).

Suggested-by: Oleg Nesterov o...@redhat.com
Signed-off-by: Anton Vorontsov anton.voront...@linaro.org
---
 arch/blackfin/kernel/trace.c |   26 +-
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index 44bbf2f..d08f0e3 100644
--- a/arch/blackfin/kernel/trace.c
+++ b/arch/blackfin/kernel/trace.c
@@ -10,6 +10,8 @@
 #include linux/hardirq.h
 #include linux/thread_info.h
 #include linux/mm.h
+#include linux/oom.h
+#include linux/sched.h
 #include linux/uaccess.h
 #include linux/module.h
 #include linux/kallsyms.h
@@ -28,7 +30,6 @@ void decode_address(char *buf, unsigned long address)
struct task_struct *p;
struct mm_struct *mm;
unsigned long flags, offset;
-   unsigned char in_atomic = (bfin_read_IPEND()  0x10) || in_atomic();
struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -114,15 +115,15 @@ void decode_address(char *buf, unsigned long address)
 */
write_lock_irqsave(tasklist_lock, flags);
for_each_process(p) {
-   mm = (in_atomic ? p-mm : get_task_mm(p));
-   if (!mm)
-   continue;
+   struct task_struct *t;
 
-   if (!down_read_trylock(mm-mmap_sem)) {
-   if (!in_atomic)
-   mmput(mm);
+   t = find_lock_task_mm(p);
+   if (!t)
continue;
-   }
+
+   mm = t-mm;
+   if (!down_read_trylock(mm-mmap_sem))
+   goto __continue;
 
for (n = rb_first(mm-mm_rb); n; n = rb_next(n)) {
struct vm_area_struct *vma;
@@ -131,7 +132,7 @@ void decode_address(char *buf, unsigned long address)
 
if (address = vma-vm_start  address  vma-vm_end) {
char _tmpbuf[256];
-   char *name = p-comm;
+   char *name = t-comm;
struct file *file = vma-vm_file;
 
if (file) {
@@ -164,8 +165,7 @@ void decode_address(char *buf, unsigned long address)
name, vma-vm_start, 
vma-vm_end);
 
up_read(mm-mmap_sem);
-   if (!in_atomic)
-   mmput(mm);
+   task_unlock(t);
 
if (buf[0] == '\0')
sprintf(buf, [ %s ] dynamic memory, 
name);
@@ -175,8 +175,8 @@ void decode_address(char *buf, unsigned long address)
}
 
up_read(mm-mmap_sem);
-   if (!in_atomic)
-   mmput(mm);
+__continue:
+   task_unlock(t);
}
 
/*
-- 
1.7.9.2

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