linux-kernel - [patch 1/2] x86/alternatives: Sync core before enabling interrupts

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Message-ID: <20231207194518.341544943@linutronix.de>
Date:   Thu,  7 Dec 2023 20:49:24 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     paul.gortmaker@...driver.com, x86@...nel.org,
        regressions@...mhuis.info, richard.purdie@...uxfoundation.org,
        regressions@...ts.linux.dev
Subject: [patch 1/2] x86/alternatives: Sync core before enabling interrupts

text_poke_early() does:

   local_irq_save(flags);
   memcpy(addr, opcode, len);
   local_irq_restore(flags);
   sync_core();

That's not really correct because the synchronization should happen before
interrupts are reenabled to ensure that a pending interrupt observes the
complete update of the opcodes.

It's not entirely clear whether the interrupt entry provides enough
serialization already, but moving the sync_core() invocation into interrupt
disabled region does no harm and is obviously correct.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/alternative.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1685,8 +1685,8 @@ void __init_or_module text_poke_early(vo
 	} else {
 		local_irq_save(flags);
 		memcpy(addr, opcode, len);
-		local_irq_restore(flags);
 		sync_core();
+		local_irq_restore(flags);
 
 		/*
 		 * Could also do a CLFLUSH here to speed up CPU recovery; but