[3.16.y-ckt stable] Patch "Correct the race condition in aarch64_insn_patch_text_sync()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    Correct the race condition in aarch64_insn_patch_text_sync()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 404bae811636a437782f1dacbb9fade3e92cec43 Mon Sep 17 00:00:00 2001
From: William Cohen <wcohen at redhat.com>
Date: Tue, 11 Nov 2014 09:41:27 -0500
Subject: Correct the race condition in aarch64_insn_patch_text_sync()

commit 899d5933b2dd2720f2b20b01eaa07871aa6ad096 upstream.

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.

Fixes: ae16480785de arm64: introduce interfaces to hotpatch kernel and module code
Signed-off-by: William Cohen <wcohen at redhat.com>
Acked-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 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 92f36835486b..565e26f23f31 100644
--- a/arch/arm64/kernel/insn.c
+++ b/arch/arm64/kernel/insn.c
@@ -156,9 +156,10 @@ 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();
 	}
--
2.1.0