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

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/alternatives branch of tip:

Commit-ID:     451283cd40bceccc02397d0e5b2b6eda6047b1ed
Gitweb:        https://git.kernel.org/tip/451283cd40bceccc02397d0e5b2b6eda6047b1ed
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 25 Mar 2025 11:30:47 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 25 Mar 2025 12:28:35 +01:00

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

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250325103047.GH36322@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/kernel/alternative.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 85089c7..5f44814 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);