Patch "[PATCH 043/135] cxl: Fix DSI misses when the context owning task" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

[PATCH 043/135] cxl: Fix DSI misses when the context owning task

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
0043-cxl-Fix-DSI-misses-when-the-context-owning-task-exit.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vaibhav Jain <[email protected]>
Date: Tue, 24 Nov 2015 16:26:18 +0530
Subject: [PATCH 043/135] cxl: Fix DSI misses when the context owning task
exits

[ Upstream commit 7b8ad495d59280b634a7b546f4cdf58cf4d65f61 ]

Presently when a user-space process issues CXL_IOCTL_START_WORK ioctl we store the pid of the current task_struct and use it to get pointer to the mm_struct of the process, while processing page or segment faults from the capi card. However this causes issues when the thread that had originally issued the start-work ioctl exits in which case the stored pid is no more valid and the cxl driver is unable to handle faults as the mm_struct corresponding to process is no more accessible.

This patch fixes this issue by using the mm_struct of the next alive task in the thread group. This is done by iterating over all the tasks in the thread group starting from thread group leader and calling get_task_mm on each one of them. When a valid mm_struct is obtained the pid of the associated task is stored in the context replacing the exiting one for handling future faults.

The patch introduces a new function named get_mem_context that checks if the current task pointed to by ctx->pid is dead? If yes it performs the steps described above. Also a new variable cxl_context.glpid is introduced which stores the pid of the thread group leader associated with the context owning task.

---
 drivers/misc/cxl/api.c     |    2 
 drivers/misc/cxl/context.c |    6 +-
 drivers/misc/cxl/cxl.h     |    3 +
 drivers/misc/cxl/fault.c   |  129 +++++++++++++++++++++++++++++++++------------
 drivers/misc/cxl/file.c    |    6 +-
 5 files changed, 109 insertions(+), 37 deletions(-)

@@ -172,7 +172,7 @@ int cxl_start_context(struct cxl_context
 
 	if (task) {
 		ctx->pid = get_task_pid(task, PIDTYPE_PID);
-		get_pid(ctx->pid);
+		ctx->glpid = get_task_pid(task->group_leader, PIDTYPE_PID);
 		kernel = false;
 	}
 

@@ -42,7 +42,7 @@ int cxl_context_init(struct cxl_context
 	spin_lock_init(&ctx->sste_lock);
 	ctx->afu = afu;
 	ctx->master = master;
-	ctx->pid = NULL; /* Set in start work ioctl */
+	ctx->pid = ctx->glpid = NULL; /* Set in start work ioctl */
 	mutex_init(&ctx->mapping_lock);
 	ctx->mapping = mapping;
 

@@ -217,7 +217,11 @@ int __detach_context(struct cxl_context
 	WARN_ON(cxl_detach_process(ctx) &&
 		cxl_adapter_link_ok(ctx->afu->adapter));
 	flush_work(&ctx->fault_work); /* Only needed for dedicated process */
+
+	/* release the reference to the group leader and mm handling pid */
 	put_pid(ctx->pid);
+	put_pid(ctx->glpid);
+
 	cxl_ctx_put();
 	return 0;
 }

@@ -445,6 +445,9 @@ struct cxl_context {
 	unsigned int sst_size, sst_lru;
 
 	wait_queue_head_t wq;
+	/* pid of the group leader associated with the pid */
+	struct pid *glpid;
+	/* use mm context associated with this pid for ds faults */
 	struct pid *pid;
 	spinlock_t lock; /* Protects pending_irq_mask, pending_fault and fault_addr */
 	/* Only used in PR mode */

@@ -201,8 +201,12 @@ static long afu_ioctl_start_work(struct
 	 * where a process (master, some daemon, etc) has opened the chardev on
 	 * behalf of another process, so the AFU's mm gets bound to the process
 	 * that performs this ioctl and not the process that opened the file.
+	 * Also we grab the PID of the group leader so that if the task that
+	 * has performed the attach operation exits the mm context of the
+	 * process is still accessible.
 	 */
-	ctx->pid = get_pid(get_task_pid(current, PIDTYPE_PID));
+	ctx->pid = get_task_pid(current, PIDTYPE_PID);
+	ctx->glpid = get_task_pid(current->group_leader, PIDTYPE_PID);
 
 	trace_cxl_attach(ctx, work.work_element_descriptor, work.num_interrupts, amr);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/0042-cxl-Fix-possible-idr-warning-when-contexts-are-relea.patch queue-4.4/0043-cxl-Fix-DSI-misses-when-the-context-owning-task-exit.patch