[PATCH] /proc/*/oom_score oops re badness

[Alexey Dobriyan <adobriyan@sw.ru>]

Eternal quest to make

	while true; do cat /proc/fs/xfs/stat >/dev/null 2>/dev/null; done
	while true; do find /proc -type f 2>/dev/null | xargs cat >/dev/null 2>/dev/null; done
	while true; do modprobe xfs; rmmod xfs; done

work reliably continues and now kernel[1] oopses in the following way:

BUG: unable to handle ... at virtual address 6b6b6b6b
EIP is at badness
process: cat
	proc_oom_score
	proc_info_read
	sys_fstat64
	vfs_read
	proc_info_read
	sys_read

Failing code is prefetch hidden in list_for_each_entry() in badness().
badness() is reachable from two points. One is proc_oom_score, another
is out_of_memory() => select_bad_process() => badness().

Second path grabs tasklist_lock, while first doesn't.

[1] 2.6.21-rc3-something + patches for all proc races I've already
    posted + patch for proxying proc directories similar to proxying
    proc regular files.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
---

 fs/proc/base.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -310,7 +310,9 @@ static int proc_oom_score(struct task_st
 	struct timespec uptime;
 
 	do_posix_clock_monotonic_gettime(&uptime);
+	read_lock(&tasklist_lock);
 	points = badness(task, uptime.tv_sec);
+	read_unlock(&tasklist_lock);
 	return sprintf(buffer, "%lu\n", points);
 }