[PATCH v5 4/4] s390/sclp: Ensure no callback gets called after sclp_unregister() returns

[Alexander Egorenkov <egorenar@linux.ibm.com>]

There is a potential race condition between sclp_unregister()
and sclp_dispatch_evbufs()/sclp_dispatch_state_change(). As a result,
it is not guaranteed that the callbacks registered with sclp_register()
will not get called one more time (and no more than one) after
sclp_unregister() returns.

The basic idea is to use a single global wait queue which is woken up
when no SCLP WRITE_EVENT_MASK and no SCLP READ_EVENT_DATA request is
outstanding. If both are true then it is guaranteed that no sclp_register
callback could be in-flight. Once the struct sclp_register given to
sclp_unregister() is removed from the list sclp_reg_list, no further
callbacks can be scheduled for the given struct sclp_register and it only
remains to wait until the SCLP driver becomes idle.

With this change sclp_unregister() may no longer be invoked from atomic
context or registered callbacks. But this represents no problem because
this is no regular use case and no driver using sclp_unregister() requires
this at the moment and likely should not require it in the future.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
---
 drivers/s390/char/sclp.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c
index 98e334724a62..c064234314bf 100644
--- a/drivers/s390/char/sclp.c
+++ b/drivers/s390/char/sclp.c
@@ -20,6 +20,7 @@
 #include <linux/jiffies.h>
 #include <linux/init.h>
 #include <linux/platform_device.h>
+#include <linux/wait.h>
 #include <asm/types.h>
 #include <asm/irq.h>
 #include <asm/debug.h>
@@ -67,6 +68,8 @@ static struct sclp_req sclp_init_req;
 static void *sclp_read_sccb;
 static struct init_sccb *sclp_init_sccb;
 
+static DECLARE_WAIT_QUEUE_HEAD(sclp_state_wq);
+
 /* Number of console pages to allocate, used by sclp_con.c and sclp_vt220.c */
 int sclp_console_pages = SCLP_CONSOLE_PAGES;
 /* Flag to indicate if buffer pages are dropped on buffer full condition */
@@ -588,6 +591,7 @@ sclp_read_cb(struct sclp_req *req, void *data)
 		sclp_dispatch_evbufs(sccb);
 	spin_lock_irqsave(&sclp_lock, flags);
 	sclp_reading_state = sclp_reading_state_idle;
+	wake_up_all(&sclp_state_wq);
 	spin_unlock_irqrestore(&sclp_lock, flags);
 }
 
@@ -898,19 +902,26 @@ sclp_register(struct sclp_register *reg)
 
 EXPORT_SYMBOL(sclp_register);
 
-/* Unregister event listener. */
-void
-sclp_unregister(struct sclp_register *reg)
+/* Unregister event listener.
+ * This function may sleep. Do not call it from atomic context or
+ * sclp_register.receive_fn(). */
+void sclp_unregister(struct sclp_register *reg)
 {
-	unsigned long flags;
+	might_sleep();
 
 	/* UREG: Event listener unregistered (b=caller) */
 	sclp_trace_register(2, "UREG", 0, _RET_IP_, reg);
 
-	spin_lock_irqsave(&sclp_lock, flags);
+	spin_lock_irq(&sclp_lock);
 	list_del(&reg->list);
-	spin_unlock_irqrestore(&sclp_lock, flags);
+	spin_unlock_irq(&sclp_lock);
 	sclp_init_mask(1);
+	spin_lock_irq(&sclp_lock);
+	wait_event_lock_irq(sclp_state_wq,
+			    (sclp_mask_state == sclp_mask_state_idle) &&
+			    (sclp_reading_state == sclp_reading_state_idle),
+			    sclp_lock);
+	spin_unlock_irq(&sclp_lock);
 }
 
 EXPORT_SYMBOL(sclp_unregister);
@@ -1036,6 +1047,7 @@ sclp_init_mask(int calculate)
 		}
 	}
 	sclp_mask_state = sclp_mask_state_idle;
+	wake_up_all(&sclp_state_wq);
 	spin_unlock_irqrestore(&sclp_lock, flags);
 	return rc;
 }
-- 
2.53.0