[PATCH] xen: take system_transition_mutex on suspend

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Xen's do_suspend() calls dpm_suspend_start() without taking required
system_transition_mutex. Since 12ffc3b1513eb moved the
pm_restrict_gfp_mask() call, not taking that mutex results in a WARN.

Take the mutex in do_suspend(), and use mutex_trylock() to follow
how enter_state() does this.

Suggested-by: Jürgen Groß <jgross@xxxxxxxx>
Fixes: 12ffc3b1513eb "PM: Restrict swap use to later in the suspend sequence"
Link: https://lore.kernel.org/xen-devel/aKiBJeqsYx_4Top5@mail-itl/
Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v6.16+
---
 drivers/xen/manage.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
index b4b4ebed68daf..f78d970949c0a 100644
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -11,6 +11,7 @@
 #include <linux/reboot.h>
 #include <linux/sysrq.h>
 #include <linux/stop_machine.h>
+#include <linux/suspend.h>
 #include <linux/freezer.h>
 #include <linux/syscore_ops.h>
 #include <linux/export.h>
@@ -101,10 +102,16 @@ static void do_suspend(void)
 
        shutting_down = SHUTDOWN_SUSPEND;
 
+       if (!mutex_trylock(&system_transition_mutex))
+       {
+               pr_err("%s: failed to take system_transition_mutex\n", 
__func__);
+               goto out;
+       }
+
        err = freeze_processes();
        if (err) {
                pr_err("%s: freeze processes failed %d\n", __func__, err);
-               goto out;
+               goto out_unlock;
        }
 
        err = freeze_kernel_threads();
@@ -160,6 +167,8 @@ static void do_suspend(void)
 
 out_thaw:
        thaw_processes();
+out_unlock:
+       mutex_unlock(&system_transition_mutex);
 out:
        shutting_down = SHUTDOWN_INVALID;
 }
-- 
2.51.0