linux-kernel - [PATCH 1/2] xen: privcmd: Switch from mutex to spinlock for irqfds

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Message-Id: <a66d7a7a9001424d432f52a9fc3931a1f345464f.1718703669.git.viresh.kumar@linaro.org>
Date: Tue, 18 Jun 2024 15:12:28 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] xen: privcmd: Switch from mutex to spinlock for irqfds

irqfd_wakeup() gets EPOLLHUP, when it is called by
eventfd_release() by way of wake_up_poll(&ctx->wqh, EPOLLHUP), which
gets called under spin_lock_irqsave(). We can't use a mutex here as it
will lead to a deadlock.

Fix it by switching over to a spin lock.

Reported-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 drivers/xen/privcmd.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 67dfa4778864..5ceb6c56cf3e 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -13,7 +13,6 @@
 #include <linux/file.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
-#include <linux/mutex.h>
 #include <linux/poll.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
@@ -845,7 +844,7 @@ static long privcmd_ioctl_mmap_resource(struct file *file,
 #ifdef CONFIG_XEN_PRIVCMD_EVENTFD
 /* Irqfd support */
 static struct workqueue_struct *irqfd_cleanup_wq;
-static DEFINE_MUTEX(irqfds_lock);
+static DEFINE_SPINLOCK(irqfds_lock);
 static LIST_HEAD(irqfds_list);
 
 struct privcmd_kernel_irqfd {
@@ -909,9 +908,11 @@ irqfd_wakeup(wait_queue_entry_t *wait, unsigned int mode, int sync, void *key)
 		irqfd_inject(kirqfd);
 
 	if (flags & EPOLLHUP) {
-		mutex_lock(&irqfds_lock);
+		unsigned long flags;
+
+		spin_lock_irqsave(&irqfds_lock, flags);
 		irqfd_deactivate(kirqfd);
-		mutex_unlock(&irqfds_lock);
+		spin_unlock_irqrestore(&irqfds_lock, flags);
 	}
 
 	return 0;
@@ -929,6 +930,7 @@ irqfd_poll_func(struct file *file, wait_queue_head_t *wqh, poll_table *pt)
 static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 {
 	struct privcmd_kernel_irqfd *kirqfd, *tmp;
+	unsigned long flags;
 	__poll_t events;
 	struct fd f;
 	void *dm_op;
@@ -968,18 +970,18 @@ static int privcmd_irqfd_assign(struct privcmd_irqfd *irqfd)
 	init_waitqueue_func_entry(&kirqfd->wait, irqfd_wakeup);
 	init_poll_funcptr(&kirqfd->pt, irqfd_poll_func);
 
-	mutex_lock(&irqfds_lock);
+	spin_lock_irqsave(&irqfds_lock, flags);
 
 	list_for_each_entry(tmp, &irqfds_list, list) {
 		if (kirqfd->eventfd == tmp->eventfd) {
 			ret = -EBUSY;
-			mutex_unlock(&irqfds_lock);
+			spin_unlock_irqrestore(&irqfds_lock, flags);
 			goto error_eventfd;
 		}
 	}
 
 	list_add_tail(&kirqfd->list, &irqfds_list);
-	mutex_unlock(&irqfds_lock);
+	spin_unlock_irqrestore(&irqfds_lock, flags);
 
 	/*
 	 * Check if there was an event already pending on the eventfd before we
@@ -1011,12 +1013,13 @@ static int privcmd_irqfd_deassign(struct privcmd_irqfd *irqfd)
 {
 	struct privcmd_kernel_irqfd *kirqfd;
 	struct eventfd_ctx *eventfd;
+	unsigned long flags;
 
 	eventfd = eventfd_ctx_fdget(irqfd->fd);
 	if (IS_ERR(eventfd))
 		return PTR_ERR(eventfd);
 
-	mutex_lock(&irqfds_lock);
+	spin_lock_irqsave(&irqfds_lock, flags);
 
 	list_for_each_entry(kirqfd, &irqfds_list, list) {
 		if (kirqfd->eventfd == eventfd) {
@@ -1025,7 +1028,7 @@ static int privcmd_irqfd_deassign(struct privcmd_irqfd *irqfd)
 		}
 	}
 
-	mutex_unlock(&irqfds_lock);
+	spin_unlock_irqrestore(&irqfds_lock, flags);
 
 	eventfd_ctx_put(eventfd);
 
@@ -1073,13 +1076,14 @@ static int privcmd_irqfd_init(void)
 static void privcmd_irqfd_exit(void)
 {
 	struct privcmd_kernel_irqfd *kirqfd, *tmp;
+	unsigned long flags;
 
-	mutex_lock(&irqfds_lock);
+	spin_lock_irqsave(&irqfds_lock, flags);
 
 	list_for_each_entry_safe(kirqfd, tmp, &irqfds_list, list)
 		irqfd_deactivate(kirqfd);
 
-	mutex_unlock(&irqfds_lock);
+	spin_unlock_irqrestore(&irqfds_lock, flags);
 
 	destroy_workqueue(irqfd_cleanup_wq);
 }
-- 
2.31.1.272.g89b43f80a514