linux-kernel - [PATCH RT 6/8] drm/vmwgfx: Drop preempt_disable() in vmw_fifo_ping

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Message-Id: <7b50a4187b38dd305b91907c235c2382cc87bca1.1583783251.git.zanussi@kernel.org>
Date:   Mon,  9 Mar 2020 14:47:51 -0500
From:   zanussi@...nel.org
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>
Subject: [PATCH RT 6/8] drm/vmwgfx: Drop preempt_disable() in vmw_fifo_ping_host()

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

v4.14.172-rt78-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


[ Upstream commit b901491e7b9b7a676818d84e482b69be72fc142f ]

vmw_fifo_ping_host() disables preemption around a test and a register
write via vmw_write(). The write function acquires a spinlock_t typed
lock which is not allowed in a preempt_disable()ed section on
PREEMPT_RT. This has been reported in the bugzilla.

It has been explained by Thomas Hellstrom that this preempt_disable()ed
section is not required for correctness.

Remove the preempt_disable() section.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206591
Link: https://lkml.kernel.org/r/0b5e1c65d89951de993deab06d1d197b40fd67aa.camel@vmware.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Tom Zanussi <zanussi@...nel.org>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c b/drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c
index a1c68e6a689e3..713f202fca2cd 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c
@@ -167,10 +167,8 @@ void vmw_fifo_ping_host(struct vmw_private *dev_priv, uint32_t reason)
 {
 	u32 *fifo_mem = dev_priv->mmio_virt;
 
-	preempt_disable();
 	if (cmpxchg(fifo_mem + SVGA_FIFO_BUSY, 0, 1) == 0)
 		vmw_write(dev_priv, SVGA_REG_SYNC, reason);
-	preempt_enable();
 }
 
 void vmw_fifo_release(struct vmw_private *dev_priv, struct vmw_fifo_state *fifo)
-- 
2.14.1