xen: take system_transition_mutex on suspend

Xen's do_suspend() calls dpm_suspend_start() without taking required
system_transition_mutex. Since 12ffc3b 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@suse.com>
Fixes: 12ffc3b "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@invisiblethingslab.com>
Cc: stable@vger.kernel.org # v6.16+
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250921162853.223116-1-marmarek@invisiblethingslab.com>

Author: marmarek

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>
@@ -95,10 +96,16 @@ 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();
@@ -155,6 +162,8 @@ static void do_suspend(void)
 
 out_thaw:
 	thaw_processes();
+out_unlock:
+	mutex_unlock(&system_transition_mutex);
 out:
 	shutting_down = SHUTDOWN_INVALID;
 }