shm: make exit_shm work proportional to task activity

This is small set of patches our team has had kicking around for a few
versions internally that fixes tasks getting hung on shm_exit when there
are many threads hammering it at once.

Anton wrote a simple test to cause the issue:

  http://ozlabs.org/~anton/junkcode/bust_shm_exit.c

Before applying this patchset, this test code will cause either hanging
tracebacks or pthread out of memory errors.

After this patchset, it will still produce output like:

  root@somehost:~# ./bust_shm_exit 1024 160
  ...
  INFO: rcu_sched detected stalls on CPUs/tasks: {} (detected by 116, t=2111 jiffies, g=241, c=240, q=7113)
  INFO: Stall ended before state dump start
  ...

But the task will continue to run along happily, so we consider this an
improvement over hanging, even if it's a bit noisy.

This patch (of 3):

exit_shm obtains the ipc_ns shm rwsem for write and holds it while it
walks every shared memory segment in the namespace.  Thus the amount of
work is related to the number of shm segments in the namespace not the
number of segments that might need to be cleaned.

In addition, this occurs after the task has been notified the thread has
exited, so the number of tasks waiting for the ns shm rwsem can grow
without bound until memory is exausted.

Add a list to the task struct of all shmids allocated by this task.  Init
the list head in copy_process.  Use the ns->rwsem for locking.  Add
segments after id is added, remove before removing from id.

On unshare of NEW_IPCNS orphan any ids as if the task had exited, similar
to handling of semaphore undo.

I chose a define for the init sequence since its a simple list init,
otherwise it would require a function call to avoid include loops between
the semaphore code and the task struct.  Converting the list_del to
list_del_init for the unshare cases would remove the exit followed by
init, but I left it blow up if not inited.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Jack Miller <millerjo@us.ibm.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 11 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index 89fc354..1fc3a61 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -178,6 +178,7 @@ static void shm_rcu_free(struct rcu_head *head)
 
 static inline void shm_rmid(struct ipc_namespace *ns, struct shmid_kernel *s)
 {
+	list_del(&s->shm_clist);
 	ipc_rmid(&shm_ids(ns), &s->shm_perm);
 }
 
@@ -268,14 +269,10 @@ static void shm_close(struct vm_area_struct *vma)
 }
 
 /* Called with ns->shm_ids(ns).rwsem locked */
-static int shm_try_destroy_current(int id, void *p, void *data)
+static void shm_mark_orphan(struct shmid_kernel *shp, struct ipc_namespace *ns)
 {
-	struct ipc_namespace *ns = data;
-	struct kern_ipc_perm *ipcp = p;
-	struct shmid_kernel *shp = container_of(ipcp, struct shmid_kernel, shm_perm);
-
-	if (shp->shm_creator != current)
-		return 0;
+	if (WARN_ON(shp->shm_creator != current)) /* Remove me when it works */
+		return;
 
 	/*
 	 * Mark it as orphaned to destroy the segment when
@@ -289,13 +286,12 @@ static int shm_try_destroy_current(int id, void *p, void *data)
 	 * is not set, it shouldn't be deleted here.
 	 */
 	if (!ns->shm_rmid_forced)
-		return 0;
+		return;
 
 	if (shm_may_destroy(ns, shp)) {
 		shm_lock_by_ptr(shp);
 		shm_destroy(ns, shp);
 	}
-	return 0;
 }
 
 /* Called with ns->shm_ids(ns).rwsem locked */
@@ -333,14 +329,17 @@ void shm_destroy_orphaned(struct ipc_namespace *ns)
 void exit_shm(struct task_struct *task)
 {
 	struct ipc_namespace *ns = task->nsproxy->ipc_ns;
+	struct shmid_kernel *shp, *n;
 
 	if (shm_ids(ns).in_use == 0)
 		return;
 
 	/* Destroy all already created segments, but not mapped yet */
 	down_write(&shm_ids(ns).rwsem);
-	if (shm_ids(ns).in_use)
-		idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_current, ns);
+	list_for_each_entry_safe(shp, n, &task->sysvshm.shm_clist, shm_clist)
+		shm_mark_orphan(shp, ns);
+	/* remove the list head from any segments still attached */
+	list_del(&task->sysvshm.shm_clist);
 	up_write(&shm_ids(ns).rwsem);
 }
 
@@ -561,6 +560,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
 	shp->shm_nattch = 0;
 	shp->shm_file = file;
 	shp->shm_creator = current;
+	list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
 
 	/*
 	 * shmid gets reported as "inode#" in /proc/pid/maps.