tools/libs/evtchn: fix locking in Mini-OS

Commit Message

Jürgen Groß Dec. 11, 2023, 7:10 a.m. UTC

When adding locking to tools/libs/evtchn/minios.c a semaphore was
used. This can result in deadlocks, as the lock is taken inside the
event handler, which can interrupt an already locked region.

The fix is rather simple, as Mini-OS is supporting a single vcpu
only. So instead of the semaphore it is enough to disable interrupts
when operating on the port list.

Fixes: bc4fe94a69d4 ("ools/libs/evtchn: replace assert()s in stubdom with proper locking")
Signed-off-by: Juergen Gross <[e-mail]>
---
 tools/libs/evtchn/minios.c | 45 +++++++++++++-------------------------
 1 file changed, 15 insertions(+), 30 deletions(-)

Comments

Anthony PERARD Dec. 13, 2023, 3:21 p.m. UTC | #1

On Mon, Dec 11, 2023 at 08:10:17AM +0100, Juergen Gross wrote:
> When adding locking to tools/libs/evtchn/minios.c a semaphore was
> used. This can result in deadlocks, as the lock is taken inside the
> event handler, which can interrupt an already locked region.

Or maybe the issue was with xenevtchn_unbind() which calls
port_dealloc() which was also trying to get the mutex.

> The fix is rather simple, as Mini-OS is supporting a single vcpu
> only. So instead of the semaphore it is enough to disable interrupts
> when operating on the port list.

Semaphore implementation does that as well, so that's fine, and it works
recursively.

> Fixes: bc4fe94a69d4 ("ools/libs/evtchn: replace assert()s in stubdom with proper locking")
> Signed-off-by: Juergen Gross <[e-mail]>


And osstest is happy with the patch:
http://logs.test-lab.xenproject.org/osstest/logs/184123/
> test-amd64-amd64-xl-qemut-stubdom-debianhvm-amd64-xsm        pass
> test-amd64-i386-xl-qemut-stubdom-debianhvm-amd64-xsm         pass

So overall, patch looks fine to me:
Acked-by: Anthony PERARD <[e-mail]>

Thanks,

Patch

diff --git a/tools/libs/evtchn/minios.c b/tools/libs/evtchn/minios.c
index c807e17f55..046cd28d37 100644
--- a/tools/libs/evtchn/minios.c
+++ b/tools/libs/evtchn/minios.c
@@ -25,7 +25,6 @@ 
 #include <mini-os/os.h>
 #include <mini-os/lib.h>
 #include <mini-os/events.h>
-#include <mini-os/semaphore.h>
 #include <mini-os/wait.h>
 
 #include <assert.h>
@@ -43,7 +42,6 @@  XEN_LIST_HEAD(port_list, struct port_info);
 
 struct ports {
     struct port_list list;
-    struct semaphore sem;
 };
 
 struct port_info {
@@ -58,6 +56,7 @@  static struct port_info *port_alloc(xenevtchn_handle *xce)
     struct port_info *port_info;
     struct file *file = get_file_from_fd(xce->fd);
     struct ports *ports = file->dev;
+    unsigned long flags;
 
     port_info = malloc(sizeof(struct port_info));
     if ( port_info == NULL )
@@ -67,19 +66,24 @@  static struct port_info *port_alloc(xenevtchn_handle *xce)
     port_info->port = -1;
     port_info->bound = false;
 
-    down(&ports->sem);
+    local_irq_save(flags);
     XEN_LIST_INSERT_HEAD(&ports->list, port_info, list);
-    up(&ports->sem);
+    local_irq_restore(flags);
 
     return port_info;
 }
 
 static void port_dealloc(struct port_info *port_info)
 {
+    unsigned long flags;
+
     if ( port_info->bound )
         unbind_evtchn(port_info->port);
 
+    local_irq_save(flags);
     XEN_LIST_REMOVE(port_info, list);
+    local_irq_restore(flags);
+
     free(port_info);
 }
 
@@ -135,7 +139,6 @@  int osdep_evtchn_open(xenevtchn_handle *xce, unsigned int flags)
 
     file->dev = ports;
     XEN_LIST_INIT(&ports->list);
-    init_SEMAPHORE(&ports->sem, 1);
     xce->fd = fd;
     printf("evtchn_open() -> %d\n", fd);
 
@@ -183,16 +186,11 @@  static void evtchn_handler(evtchn_port_t port, struct pt_regs *regs, void *data)
     ports = file->dev;
     mask_evtchn(port);
 
-    down(&ports->sem);
     XEN_LIST_FOREACH(port_info, &ports->list, list)
     {
         if ( port_info->port == port )
-        {
-            up(&ports->sem);
             goto found;
-        }
     }
-    up(&ports->sem);
 
     printk("Unknown port %d for handle %d\n", port, xce->fd);
     return;
@@ -203,16 +201,6 @@  static void evtchn_handler(evtchn_port_t port, struct pt_regs *regs, void *data)
     wake_up(&event_queue);
 }
 
-static void port_remove(xenevtchn_handle *xce, struct port_info *port_info)
-{
-    struct file *file = get_file_from_fd(xce->fd);
-    struct ports *ports = file->dev;
-
-    down(&ports->sem);
-    port_dealloc(port_info);
-    up(&ports->sem);
-}
-
 xenevtchn_port_or_error_t xenevtchn_bind_unbound_port(xenevtchn_handle *xce,
                                                       uint32_t domid)
 {
@@ -230,7 +218,7 @@  xenevtchn_port_or_error_t xenevtchn_bind_unbound_port(xenevtchn_handle *xce,
 
     if ( ret < 0 )
     {
-        port_remove(xce, port_info);
+        port_dealloc(port_info);
         errno = -ret;
         return -1;
     }
@@ -261,7 +249,7 @@  xenevtchn_port_or_error_t xenevtchn_bind_interdomain(xenevtchn_handle *xce,
 
     if ( ret < 0 )
     {
-        port_remove(xce, port_info);
+        port_dealloc(port_info);
         errno = -ret;
         return -1;
     }
@@ -279,18 +267,19 @@  int xenevtchn_unbind(xenevtchn_handle *xce, evtchn_port_t port)
     struct file *file = get_file_from_fd(fd);
     struct port_info *port_info;
     struct ports *ports = file->dev;
+    unsigned long flags;
 
-    down(&ports->sem);
+    local_irq_save(flags);
     XEN_LIST_FOREACH(port_info, &ports->list, list)
     {
         if ( port_info->port == port )
         {
             port_dealloc(port_info);
-            up(&ports->sem);
+            local_irq_restore(flags);
             return 0;
         }
     }
-    up(&ports->sem);
+    local_irq_restore(flags);
 
     printf("Warning: couldn't find port %"PRId32" for xc handle %x\n",
            port, fd);
@@ -315,7 +304,7 @@  xenevtchn_port_or_error_t xenevtchn_bind_virq(xenevtchn_handle *xce,
 
     if ( port < 0 )
     {
-        port_remove(xce, port_info);
+        port_dealloc(port_info);
         errno = -port;
         return -1;
     }
@@ -335,8 +324,6 @@  xenevtchn_port_or_error_t xenevtchn_pending(xenevtchn_handle *xce)
     unsigned long flags;
     evtchn_port_t ret = -1;
 
-    down(&ports->sem);
-
     local_irq_save(flags);
 
     file->read = false;
@@ -360,8 +347,6 @@  xenevtchn_port_or_error_t xenevtchn_pending(xenevtchn_handle *xce)
 
     local_irq_restore(flags);
 
-    up(&ports->sem);
-
     return ret;
 }