Hi,
Can you review this patch? I'm new to locking in the Linux kernel
so I may be misssing something.
I think we must disable IRQs between relay_reserve and initializing
the data; consider the following scenario where task 1 and task 2
runs on the same CPU:
task 1: trace_note_message task 2: interrupt userspace (blktrace)
-------------------------- ----------------- --------------------
__trace_note_message read(relay)
relay_reserve <blocks ...>
<interrupted: I/O completion>
__blk_add_trace
relay_reserve
<buffers switched,
wake user>
<reads uninitialized
trace_note_message>
<done>
<runs again>
memcpy() - too late
--
Carl Henrik
>From 30fce97a2d7c02ba265eceed59592dbdc9c34f26 Mon Sep 17 00:00:00 2001
From: Carl Henrik Lunde <chlunde@xxxxxxxxxxx>
Date: Fri, 30 May 2008 12:57:47 +0200
Subject: [PATCH] block: disable IRQs until data is written to relay channel
As we may run relay_reserve from interrupt context we must always disable
IRQs. This is because a call to relay_reserve may expose previously written
data to use space.
Updated new message code and an old but related comment.
Signed-off-by: Carl Henrik Lunde <chlunde@xxxxxxxxxxx>
---
block/blktrace.c | 10 ++++------
1 files changed, 4 insertions(+), 6 deletions(-)
diff --git a/block/blktrace.c b/block/blktrace.c
index 7ae87cc..8d3a277 100644
--- a/block/blktrace.c
+++ b/block/blktrace.c
@@ -79,16 +79,17 @@ void __trace_note_message(struct blk_trace *bt, const char *fmt, ...)
{
int n;
va_list args;
+ unsigned long flags;
char *buf;
- preempt_disable();
+ local_irq_save(flags);
buf = per_cpu_ptr(bt->msg_data, smp_processor_id());
va_start(args, fmt);
n = vscnprintf(buf, BLK_TN_MAX_MSG, fmt, args);
va_end(args);
trace_note(bt, 0, BLK_TN_MESSAGE, buf, n);
- preempt_enable();
+ local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(__trace_note_message);
@@ -158,10 +159,7 @@ void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
/*
* A word about the locking here - we disable interrupts to reserve
* some space in the relay per-cpu buffer, to prevent an irq
- * from coming in and stepping on our toes. Once reserved, it's
- * enough to get preemption disabled to prevent read of this data
- * before we are through filling it. get_cpu()/put_cpu() does this
- * for us
+ * from coming in and stepping on our toes.
*/
local_irq_save(flags);
--
1.5.4.3
