From 6a20dbd6caa2358716136144bf524331d70b1e03 Mon Sep 17 00:00:00 2001
From: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Date: Tue, 8 Apr 2014 14:42:04 +0200
Subject: tty: Fix race condition between __tty_buffer_request_room and
flush_to_ldisc
The race was introduced while development of linux-3.11 by
e8437d7ecbc50198705331449367d401ebb3181f and
e9975fdec0138f1b2a85b9624e41660abd9865d4.
Originally it was found and reproduced on linux-3.12.15 and
linux-3.12.15-rt25, by sending 500 byte blocks with 115kbaud to the
target uart in a loop with 100 milliseconds delay.
In short:
1. The consumer flush_to_ldisc is on to remove the head tty_buffer.
2. The producer adds a number of bytes, so that a new tty_buffer must
be allocated and added by __tty_buffer_request_room.
3. The consumer removes the head tty_buffer element, without handling
newly committed data.
Detailed example:
* Initial buffer:
* Head, Tail -> 0: used=250; commit=250; read=240; next=NULL
* Consumer: ''flush_to_ldisc''
* consumed 10 Byte
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
{{{
count = head->commit - head->read; // count = 0
if (!count) { // enter
// INTERRUPTED BY PRODUCER ->
if (head->next == NULL)
break;
buf->head = head->next;
tty_buffer_free(port, head);
continue;
}
}}}
* Producer: tty_insert_flip_... 10 bytes + tty_flip_buffer_push
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
* added 6 bytes: head-element filled to maximum.
* buffer:
* Head, Tail -> 0: used=256; commit=250; read=250; next=NULL
* added 4 bytes: __tty_buffer_request_room is called
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=0; read=250 next=NULL
* push (tty_flip_buffer_push)
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=4; read=250 next=NULL
* Consumer
{{{
count = head->commit - head->read;
if (!count) {
// INTERRUPTED BY PRODUCER <-
if (head->next == NULL) // -> no break
break;
buf->head = head->next;
tty_buffer_free(port, head);
// ERROR: tty_buffer head freed -> 6 bytes lost
continue;
}
}}}
This patch reintroduces a spin_lock to protect this case. Perhaps later
a lock-less solution could be found.
Signed-off-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Cc: stable <stable@vger.kernel.org> # 3.11
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/tty_buffer.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
(limited to 'drivers/tty/tty_buffer.c')
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 8ebd9f88a6f6..f1d30f6945af 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -255,11 +255,16 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
if (change || left < size) {
/* This is the slow path - looking for new buffers to use */
if ((n = tty_buffer_alloc(port, size)) != NULL) {
+ unsigned long iflags;
+
n->flags = flags;
buf->tail = n;
+
+ spin_lock_irqsave(&buf->flush_lock, iflags);
b->commit = b->used;
- smp_mb();
b->next = n;
+ spin_unlock_irqrestore(&buf->flush_lock, iflags);
+
} else if (change)
size = 0;
else
@@ -443,6 +448,7 @@ static void flush_to_ldisc(struct work_struct *work)
mutex_lock(&buf->lock);
while (1) {
+ unsigned long flags;
struct tty_buffer *head = buf->head;
int count;
@@ -450,14 +456,19 @@ static void flush_to_ldisc(struct work_struct *work)
if (atomic_read(&buf->priority))
break;
+ spin_lock_irqsave(&buf->flush_lock, flags);
count = head->commit - head->read;
if (!count) {
- if (head->next == NULL)
+ if (head->next == NULL) {
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
break;
+ }
buf->head = head->next;
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
tty_buffer_free(port, head);
continue;
}
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
count = receive_buf(tty, head, count);
if (!count)
@@ -512,6 +523,7 @@ void tty_buffer_init(struct tty_port *port)
struct tty_bufhead *buf = &port->buf;
mutex_init(&buf->lock);
+ spin_lock_init(&buf->flush_lock);
tty_buffer_reset(&buf->sentinel, 0);
buf->head = &buf->sentinel;
buf->tail = &buf->sentinel;
--
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