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From:   Mitchell Levy via B4 Relay 
        <devnull+levymitchell0.gmail.com@kernel.org>
Date:   Wed, 09 Aug 2023 22:36:32 +0000
Subject: [PATCH RFC 1/2] Use raw_spinlock_t for vmbus_channel.sched_lock
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Message-Id: <20230809-b4-rt_preempt-fix-v1-1-7283bbdc8b14@gmail.com>
References: <20230809-b4-rt_preempt-fix-v1-0-7283bbdc8b14@gmail.com>
In-Reply-To: <20230809-b4-rt_preempt-fix-v1-0-7283bbdc8b14@gmail.com>
To:     "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        =?utf-8?q?Krzysztof_Wilczy=C5=84ski?= <kw@linux.com>,
        Rob Herring <robh@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>
Cc:     linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org, Mitchell Levy <levymitchell0@gmail.com>
Reply-To: <levymitchell0@gmail.com>
Precedence: bulk

From: Mitchell Levy <levymitchell0@gmail.com>

In vmbus_drv.c, vmbus_channel.sched_lock is acquired while holding an
rcu_read_lock. Since this is not a sleepable context, change sched_lock
to be a raw_spinlock_t to avoid sleeping when PREEMPT_RT is enabled.
---
 drivers/hv/channel.c                | 4 ++--
 drivers/hv/channel_mgmt.c           | 2 +-
 drivers/hv/vmbus_drv.c              | 4 ++--
 drivers/pci/controller/pci-hyperv.c | 6 +++---
 include/linux/hyperv.h              | 2 +-
 5 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
index 56f7e06c673e..00d73ec060ed 100644
--- a/drivers/hv/channel.c
+++ b/drivers/hv/channel.c
@@ -910,9 +910,9 @@ void vmbus_reset_channel_cb(struct vmbus_channel *channel)
 	tasklet_disable(&channel->callback_event);
 
 	/* See the inline comments in vmbus_chan_sched(). */
-	spin_lock_irqsave(&channel->sched_lock, flags);
+	raw_spin_lock_irqsave(&channel->sched_lock, flags);
 	channel->onchannel_callback = NULL;
-	spin_unlock_irqrestore(&channel->sched_lock, flags);
+	raw_spin_unlock_irqrestore(&channel->sched_lock, flags);
 
 	channel->sc_creation_callback = NULL;
 
diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 2f4d09ce027a..7679e6a3a645 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -348,7 +348,7 @@ static struct vmbus_channel *alloc_channel(void)
 	if (!channel)
 		return NULL;
 
-	spin_lock_init(&channel->sched_lock);
+	raw_spin_lock_init(&channel->sched_lock);
 	init_completion(&channel->rescind_event);
 
 	INIT_LIST_HEAD(&channel->sc_list);
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 67f95a29aeca..b865d00c46dc 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1257,7 +1257,7 @@ static void vmbus_chan_sched(struct hv_per_cpu_context *hv_cpu)
 		 * sched_lock critical section.  See also the inline comments
 		 * in vmbus_reset_channel_cb().
 		 */
-		spin_lock(&channel->sched_lock);
+		raw_spin_lock(&channel->sched_lock);
 
 		callback_fn = channel->onchannel_callback;
 		if (unlikely(callback_fn == NULL))
@@ -1280,7 +1280,7 @@ static void vmbus_chan_sched(struct hv_per_cpu_context *hv_cpu)
 		}
 
 sched_unlock:
-		spin_unlock(&channel->sched_lock);
+		raw_spin_unlock(&channel->sched_lock);
 sched_unlock_rcu:
 		rcu_read_unlock();
 	}
diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 2d93d0c4f10d..9ede3be05782 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1995,13 +1995,13 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 		 * sched_lock critical section.  See also the inline comments
 		 * in vmbus_reset_channel_cb().
 		 */
-		spin_lock_irqsave(&channel->sched_lock, flags);
+		raw_spin_lock_irqsave(&channel->sched_lock, flags);
 		if (unlikely(channel->onchannel_callback == NULL)) {
-			spin_unlock_irqrestore(&channel->sched_lock, flags);
+			raw_spin_unlock_irqrestore(&channel->sched_lock, flags);
 			goto enable_tasklet;
 		}
 		hv_pci_onchannelcallback(hbus);
-		spin_unlock_irqrestore(&channel->sched_lock, flags);
+		raw_spin_unlock_irqrestore(&channel->sched_lock, flags);
 
 		udelay(100);
 	}
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 3ac3974b3c78..56a1fb1647a4 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -873,7 +873,7 @@ struct vmbus_channel {
 	 * Synchronize channel scheduling and channel removal; see the inline
 	 * comments in vmbus_chan_sched() and vmbus_reset_channel_cb().
 	 */
-	spinlock_t sched_lock;
+	raw_spinlock_t sched_lock;
 
 	/*
 	 * A channel can be marked for one of three modes of reading:

-- 
2.25.1