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n_tty: Remove echo_lock
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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@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Peter Hurley authored and Greg Kroah-Hartman committed Jul 24, 2013
1 parent 862eeff commit 17bd790
Showing 1 changed file with 28 additions and 47 deletions.
75 changes: 28 additions & 47 deletions drivers/tty/n_tty.c
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Original file line number Diff line number Diff line change
Expand Up @@ -106,18 +106,17 @@ 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;
unsigned int canon_column;

struct mutex atomic_read_lock;
struct mutex output_lock;
struct mutex echo_lock;
};

static inline size_t read_cnt(struct n_tty_data *ldata)
Expand Down Expand Up @@ -333,10 +332,7 @@ 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);
Expand Down Expand Up @@ -642,8 +638,7 @@ static ssize_t process_output_block(struct tty_struct *tty,
* 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)
Expand All @@ -653,16 +648,15 @@ 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) {
Expand Down Expand Up @@ -779,27 +773,38 @@ 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
* @ldata: n_tty data
*
* 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.
Expand All @@ -811,6 +816,7 @@ 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;
Expand All @@ -821,16 +827,12 @@ 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);
}

/**
Expand All @@ -839,16 +841,12 @@ 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);
}

/**
Expand All @@ -864,15 +862,11 @@ 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);

Expand All @@ -884,8 +878,6 @@ 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);
}

/**
Expand All @@ -897,20 +889,16 @@ 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);
}

/**
Expand All @@ -923,16 +911,12 @@ 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);
Expand All @@ -941,8 +925,6 @@ 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);
}

/**
Expand Down Expand Up @@ -1284,7 +1266,7 @@ 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);
Expand All @@ -1295,7 +1277,7 @@ 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)) {
Expand Down Expand Up @@ -1326,7 +1308,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
start_tty(tty);
if (L_ECHO(tty)) {
echo_char(c, tty);
process_echoes(tty);
commit_echoes(tty);
}
isig(signal, tty);
return;
Expand All @@ -1345,7 +1327,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
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)) {
Expand All @@ -1355,7 +1337,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
if (L_ECHOCTL(tty)) {
echo_char_raw('^', ldata);
echo_char_raw('\b', ldata);
process_echoes(tty);
commit_echoes(tty);
}
}
return;
Expand All @@ -1371,7 +1353,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
echo_char(read_buf(ldata, tail), tty);
tail++;
}
process_echoes(tty);
commit_echoes(tty);
return;
}
if (c == '\n') {
Expand All @@ -1382,7 +1364,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
}
if (L_ECHO(tty) || L_ECHONL(tty)) {
echo_char_raw('\n', ldata);
process_echoes(tty);
commit_echoes(tty);
}
goto handle_newline;
}
Expand Down Expand Up @@ -1411,7 +1393,7 @@ 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);
}
/*
* XXX does PARMRK doubling happen for
Expand Down Expand Up @@ -1448,7 +1430,7 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
echo_set_canon_col(ldata);
echo_char(c, tty);
}
process_echoes(tty);
commit_echoes(tty);
}

if (parmrk)
Expand Down Expand Up @@ -1713,7 +1695,6 @@ 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);
Expand Down

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