>From 41c728aeee2185fd30ec6a8ba223a2caec875f47 Mon Sep 17 00:00:00 2001 From: William Cohen Date: Tue, 11 Nov 2014 09:41:27 -0500 Subject: [PATCH] Correct the race condition in aarch64_insn_patch_text_sync() When experimenting with patches to provide kprobes support for aarch64 smp machines would hang when inserting breakpoints into kernel code. The hangs were caused by a race condition in the code called by aarch64_insn_patch_text_sync(). The first processor in the aarch64_insn_patch_text_cb() function would patch the code while other processors were still entering the function and incrementing the cpu_count field. This resulted in some processors never observing the exit condition and exiting the function. Thus, processors in the system hung. The first processor to enter the patching function performs the patching and signals that the patching is complete with an increment of the cpu_count field. When all the processors have incremented the cpu_count field the cpu_count will be num_cpus_online()+1 and they will return to normal execution. Signed-off-by: William Cohen --- arch/arm64/kernel/insn.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c index e007714..8cd27fe 100644 --- a/arch/arm64/kernel/insn.c +++ b/arch/arm64/kernel/insn.c @@ -163,9 +163,10 @@ static int __kprobes aarch64_insn_patch_text_cb(void *arg) * which ends with "dsb; isb" pair guaranteeing global * visibility. */ - atomic_set(&pp->cpu_count, -1); + /* Notify other processors with an additional increment. */ + atomic_inc(&pp->cpu_count); } else { - while (atomic_read(&pp->cpu_count) != -1) + while (atomic_read(&pp->cpu_count) <= num_online_cpus()) cpu_relax(); isb(); } -- 1.8.3.1