From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 11 Apr 2025 05:40:14 +0000 (+0200)
Subject: x86/alternatives: Document the text_poke_bp_batch() synchronization rules a bit more
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=d60e4b2410e1b9f7c5ca347c78c6b07175c2e873;p=users%2Fdwmw2%2Flinux.git

x86/alternatives: Document the text_poke_bp_batch() synchronization rules a bit more

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20250411054105.2341982-3-mingo@kernel.org
---

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 85089c79a828e..5f448142aa993 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -2751,6 +2751,13 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 
 	/*
 	 * Remove and wait for refs to be zero.
+	 *
+	 * Notably, if after step-3 above the INT3 got removed, then the
+	 * text_poke_sync() will have serialized against any running INT3
+	 * handlers and the below spin-wait will not happen.
+	 *
+	 * IOW. unless the replacement instruction is INT3, this case goes
+	 * unused.
 	 */
 	for_each_possible_cpu(i) {
 		atomic_t *refs = per_cpu_ptr(&bp_refs, i);