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Date: Sat, 15 Jun 2013 10:04:24 -0400
From: Peter Hurley <peter@...leysoftware.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
Jiri Slaby <jslaby@...e.cz>,
Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH v2 4/9] n_tty: Remove echo_lock
Adding data to echo_buf (via add_echo_byte()) is guaranteed to be
single-threaded, since all callers are from the n_tty_receive_buf()
path. Processing the echo_buf can be called from either the
n_tty_receive_buf() path or the n_tty_write() path; however, these
callers are already serialized by output_lock.
Publish cumulative echo_head changes to echo_commit; process echo_buf
from echo_tail to echo_commit; remove echo_lock.
On echo_buf overrun, claim output_lock to serialize changes to
echo_tail.
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
drivers/tty/n_tty.c | 75 ++++++++++++++++++++---------------------------------
1 file changed, 28 insertions(+), 47 deletions(-)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 68dc9fd..d22c14c 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -106,10 +106,10 @@ struct n_tty_data {
/* consumer-published */
size_t read_tail;
- /* protected by echo_lock */
unsigned char *echo_buf;
size_t echo_head;
size_t echo_tail;
+ size_t echo_commit;
/* protected by output lock */
unsigned int column;
@@ -117,7 +117,6 @@ struct n_tty_data {
struct mutex atomic_read_lock;
struct mutex output_lock;
- struct mutex echo_lock;
};
static inline size_t read_cnt(struct n_tty_data *ldata)
@@ -332,10 +331,7 @@ static void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
static void reset_buffer_flags(struct n_tty_data *ldata)
{
ldata->read_head = ldata->canon_head = ldata->read_tail = 0;
-
- mutex_lock(&ldata->echo_lock);
- ldata->echo_head = ldata->echo_tail = 0;
- mutex_unlock(&ldata->echo_lock);
+ ldata->echo_head = ldata->echo_tail = ldata->echo_commit = 0;
ldata->erasing = 0;
bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
@@ -641,8 +637,7 @@ break_out:
* are prioritized. Also, when control characters are echoed with a
* prefixed "^", the pair is treated atomically and thus not separated.
*
- * Locking: output_lock to protect column state and space left,
- * echo_lock to protect the echo buffer
+ * Locking: output_lock to protect column state and space left
*/
static void process_echoes(struct tty_struct *tty)
@@ -652,16 +647,15 @@ static void process_echoes(struct tty_struct *tty)
size_t tail;
unsigned char c;
- if (ldata->echo_head == ldata->echo_tail)
+ if (ldata->echo_commit == ldata->echo_tail)
return;
mutex_lock(&ldata->output_lock);
- mutex_lock(&ldata->echo_lock);
space = tty_write_room(tty);
tail = ldata->echo_tail;
- nr = ldata->echo_head - ldata->echo_tail;
+ nr = ldata->echo_commit - ldata->echo_tail;
while (nr > 0) {
c = echo_buf(ldata, tail);
if (c == ECHO_OP_START) {
@@ -778,13 +772,21 @@ static void process_echoes(struct tty_struct *tty)
ldata->echo_tail = tail;
- mutex_unlock(&ldata->echo_lock);
mutex_unlock(&ldata->output_lock);
if (tty->ops->flush_chars)
tty->ops->flush_chars(tty);
}
+static void commit_echoes(struct tty_struct *tty)
+{
+ struct n_tty_data *ldata = tty->disc_data;
+
+ smp_mb();
+ ldata->echo_commit = ldata->echo_head;
+ process_echoes(tty);
+}
+
/**
* add_echo_byte - add a byte to the echo buffer
* @c: unicode byte to echo
@@ -792,13 +794,16 @@ static void process_echoes(struct tty_struct *tty)
*
* Add a character or operation byte to the echo buffer.
*
- * Should be called under the echo lock to protect the echo buffer.
+ * Locks: may claim output_lock to prevent concurrent modify of
+ * echo_tail by process_echoes().
*/
static void add_echo_byte(unsigned char c, struct n_tty_data *ldata)
{
if (ldata->echo_head - ldata->echo_tail == N_TTY_BUF_SIZE) {
size_t head = ldata->echo_head;
+
+ mutex_lock(&ldata->output_lock);
/*
* Since the buffer start position needs to be advanced,
* be sure to step by a whole operation byte group.
@@ -810,6 +815,7 @@ static void add_echo_byte(unsigned char c, struct n_tty_data *ldata)
ldata->echo_tail += 2;
} else
ldata->echo_tail++;
+ mutex_unlock(&ldata->output_lock);
}
*echo_buf_addr(ldata, ldata->echo_head++) = c;
@@ -820,16 +826,12 @@ static void add_echo_byte(unsigned char c, struct n_tty_data *ldata)
* @ldata: n_tty data
*
* Add an operation to the echo buffer to move back one column.
- *
- * Locking: echo_lock to protect the echo buffer
*/
static void echo_move_back_col(struct n_tty_data *ldata)
{
- mutex_lock(&ldata->echo_lock);
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(ECHO_OP_MOVE_BACK_COL, ldata);
- mutex_unlock(&ldata->echo_lock);
}
/**
@@ -838,16 +840,12 @@ static void echo_move_back_col(struct n_tty_data *ldata)
*
* Add an operation to the echo buffer to set the canon column
* to the current column.
- *
- * Locking: echo_lock to protect the echo buffer
*/
static void echo_set_canon_col(struct n_tty_data *ldata)
{
- mutex_lock(&ldata->echo_lock);
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(ECHO_OP_SET_CANON_COL, ldata);
- mutex_unlock(&ldata->echo_lock);
}
/**
@@ -863,15 +861,11 @@ static void echo_set_canon_col(struct n_tty_data *ldata)
* of input. This information will be used later, along with
* canon column (if applicable), to go back the correct number
* of columns.
- *
- * Locking: echo_lock to protect the echo buffer
*/
static void echo_erase_tab(unsigned int num_chars, int after_tab,
struct n_tty_data *ldata)
{
- mutex_lock(&ldata->echo_lock);
-
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(ECHO_OP_ERASE_TAB, ldata);
@@ -883,8 +877,6 @@ static void echo_erase_tab(unsigned int num_chars, int after_tab,
num_chars |= 0x80;
add_echo_byte(num_chars, ldata);
-
- mutex_unlock(&ldata->echo_lock);
}
/**
@@ -896,20 +888,16 @@ static void echo_erase_tab(unsigned int num_chars, int after_tab,
* L_ECHO(tty) is true. Called from the driver receive_buf path.
*
* This variant does not treat control characters specially.
- *
- * Locking: echo_lock to protect the echo buffer
*/
static void echo_char_raw(unsigned char c, struct n_tty_data *ldata)
{
- mutex_lock(&ldata->echo_lock);
if (c == ECHO_OP_START) {
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(ECHO_OP_START, ldata);
} else {
add_echo_byte(c, ldata);
}
- mutex_unlock(&ldata->echo_lock);
}
/**
@@ -922,16 +910,12 @@ static void echo_char_raw(unsigned char c, struct n_tty_data *ldata)
*
* This variant tags control characters to be echoed as "^X"
* (where X is the letter representing the control char).
- *
- * Locking: echo_lock to protect the echo buffer
*/
static void echo_char(unsigned char c, struct tty_struct *tty)
{
struct n_tty_data *ldata = tty->disc_data;
- mutex_lock(&ldata->echo_lock);
-
if (c == ECHO_OP_START) {
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(ECHO_OP_START, ldata);
@@ -940,8 +924,6 @@ static void echo_char(unsigned char c, struct tty_struct *tty)
add_echo_byte(ECHO_OP_START, ldata);
add_echo_byte(c, ldata);
}
-
- mutex_unlock(&ldata->echo_lock);
}
/**
@@ -1283,7 +1265,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
if (ldata->canon_head == ldata->read_head)
echo_set_canon_col(ldata);
echo_char(c, tty);
- process_echoes(tty);
+ commit_echoes(tty);
}
if (parmrk)
put_tty_queue(c, ldata);
@@ -1294,7 +1276,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
if (I_IXON(tty)) {
if (c == START_CHAR(tty)) {
start_tty(tty);
- process_echoes(tty);
+ commit_echoes(tty);
return;
}
if (c == STOP_CHAR(tty)) {
@@ -1325,7 +1307,7 @@ send_signal:
start_tty(tty);
if (L_ECHO(tty)) {
echo_char(c, tty);
- process_echoes(tty);
+ commit_echoes(tty);
}
isig(signal, tty);
return;
@@ -1344,7 +1326,7 @@ send_signal:
if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
(c == WERASE_CHAR(tty) && L_IEXTEN(tty))) {
eraser(c, tty);
- process_echoes(tty);
+ commit_echoes(tty);
return;
}
if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
@@ -1354,7 +1336,7 @@ send_signal:
if (L_ECHOCTL(tty)) {
echo_char_raw('^', ldata);
echo_char_raw('\b', ldata);
- process_echoes(tty);
+ commit_echoes(tty);
}
}
return;
@@ -1370,7 +1352,7 @@ send_signal:
echo_char(read_buf(ldata, tail), tty);
tail++;
}
- process_echoes(tty);
+ commit_echoes(tty);
return;
}
if (c == '\n') {
@@ -1381,7 +1363,7 @@ send_signal:
}
if (L_ECHO(tty) || L_ECHONL(tty)) {
echo_char_raw('\n', ldata);
- process_echoes(tty);
+ commit_echoes(tty);
}
goto handle_newline;
}
@@ -1410,7 +1392,7 @@ send_signal:
if (ldata->canon_head == ldata->read_head)
echo_set_canon_col(ldata);
echo_char(c, tty);
- process_echoes(tty);
+ commit_echoes(tty);
}
/*
* XXX does PARMRK doubling happen for
@@ -1447,7 +1429,7 @@ handle_newline:
echo_set_canon_col(ldata);
echo_char(c, tty);
}
- process_echoes(tty);
+ commit_echoes(tty);
}
if (parmrk)
@@ -1712,7 +1694,6 @@ static int n_tty_open(struct tty_struct *tty)
ldata->overrun_time = jiffies;
mutex_init(&ldata->atomic_read_lock);
mutex_init(&ldata->output_lock);
- mutex_init(&ldata->echo_lock);
/* These are ugly. Currently a malloc failure here can panic */
ldata->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
--
1.8.1.2
--
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