linux-kernel - [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq

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Date:   Tue, 21 Sep 2021 09:02:26 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()

Disabling preemption in xen_irq_enable() is not needed. There is no
risk of missing events due to preemption, as preemption can happen
only in case an event is being received, which is just the opposite
of missing an event.

Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/xen/irq.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/arch/x86/xen/irq.c b/arch/x86/xen/irq.c
index dfa091d79c2e..ba9b14a97109 100644
--- a/arch/x86/xen/irq.c
+++ b/arch/x86/xen/irq.c
@@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
 {
 	struct vcpu_info *vcpu;
 
-	/*
-	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
-	 * cleared, so disable preemption to ensure we check for
-	 * events on the VCPU we are still running on.
-	 */
-	preempt_disable();
-
 	vcpu = this_cpu_read(xen_vcpu);
 	vcpu->evtchn_upcall_mask = 0;
 
-	/* Doesn't matter if we get preempted here, because any
-	   pending event will get dealt with anyway. */
+	/*
+	 * Now preemption could happen, but this is only possible if an event
+	 * was handled, so missing an event due to preemption is not
+	 * possible at all.
+	 * The worst possible case is to be preempted and then check events
+	 * pending on the old vcpu, but this is not problematic.
+	 */
 
 	barrier(); /* unmask then check (avoid races) */
 	if (unlikely(vcpu->evtchn_upcall_pending))
 		xen_force_evtchn_callback();
-
-	preempt_enable();
 }
 PV_CALLEE_SAVE_REGS_THUNK(xen_irq_enable);
 
-- 
2.26.2