[PATCH 12/12] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files

[Michel Lespinasse <walken@google.com>]

This helps in the following situation:
- Thread A takes a page fault while reading or writing memory.
  do_page_fault() acquires the mmap_sem for read and blocks on disk
  (either reading the page from file, or hitting swap) for a long time.
- Thread B does an mmap call and blocks trying to acquire the mmap_sem
  for write
- Thread C is a monitoring process trying to read every /proc/pid/maps
  in the system. This requires acquiring the mmap_sem for read. Thread C
  blocks behind B, waiting for A to release the rwsem.  If thread C
  could be allowed to run in parallel with A, it would probably get done
  long before thread A's disk access completes, thus not actually slowing
  down thread B.

Test results with down_read_unfair_test (10 seconds):

2.6.33.3:
threadA completes ~600 faults
threadB completes ~300 mmap/munmap cycles
threadC completes ~600 /proc/pid/maps reads

2.6.33.3 + down_read_unfair:
threadA completes ~600 faults
threadB completes ~300 mmap/munmap cycles
threadC completes ~160000 /proc/pid/maps reads

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 fs/proc/base.c       |    2 +-
 fs/proc/task_mmu.c   |    2 +-
 fs/proc/task_nommu.c |    2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 8418fcc..9132488 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1367,7 +1367,7 @@ struct file *get_mm_exe_file(struct mm_struct *mm)
 
 	/* We need mmap_sem to protect against races with removal of
 	 * VM_EXECUTABLE vmas */
-	down_read(&mm->mmap_sem);
+	down_read_unfair(&mm->mmap_sem);
 	exe_file = mm->exe_file;
 	if (exe_file)
 		get_file(exe_file);
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 0705534..09647ad 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -123,7 +123,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	mm = mm_for_maps(priv->task);
 	if (!mm)
 		return NULL;
-	down_read(&mm->mmap_sem);
+	down_read_unfair(&mm->mmap_sem);
 
 	tail_vma = get_gate_vma(priv->task);
 	priv->tail_vma = tail_vma;
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 46d4b5d..56ca830 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -194,7 +194,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 		priv->task = NULL;
 		return NULL;
 	}
-	down_read(&mm->mmap_sem);
+	down_read_unfair(&mm->mmap_sem);
 
 	/* start from the Nth VMA */
 	for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
-- 
1.7.0.1