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tty: Use raw spin lock to protect the TTY read section
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The "normal" spin lock that guards the N_TTY line discipline read section
is replaced by a raw spin lock.

On a PREEMP_RT system this prevents unwanted scheduling overhead when data is
read at the same time as data is being received: while RX IRQ threaded handling
is busy a TTY read call is performed from a RT priority > threaded IRQ priority.
The read call tries to take the read section spin lock (held by the threaded
IRQ) which blocks and causes a context switch to/from the threaded IRQ handler
until the spin lock is unlocked.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Ivo Sieben authored and Greg Kroah-Hartman committed Feb 4, 2013
1 parent 183d95c commit 9800121
Showing 1 changed file with 28 additions and 28 deletions.
56 changes: 28 additions & 28 deletions drivers/tty/n_tty.c
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Original file line number Diff line number Diff line change
Expand Up @@ -100,7 +100,7 @@ struct n_tty_data {
struct mutex atomic_read_lock;
struct mutex output_lock;
struct mutex echo_lock;
spinlock_t read_lock;
raw_spinlock_t read_lock;
};

static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
Expand Down Expand Up @@ -182,9 +182,9 @@ static void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
* The problem of stomping on the buffers ends here.
* Why didn't anyone see this one coming? --AJK
*/
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
put_tty_queue_nolock(c, ldata);
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
}

/**
Expand Down Expand Up @@ -218,9 +218,9 @@ static void reset_buffer_flags(struct tty_struct *tty)
struct n_tty_data *ldata = tty->disc_data;
unsigned long flags;

spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = ldata->read_tail = ldata->read_cnt = 0;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);

mutex_lock(&ldata->echo_lock);
ldata->echo_pos = ldata->echo_cnt = ldata->echo_overrun = 0;
Expand Down Expand Up @@ -276,15 +276,15 @@ static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
unsigned long flags;
ssize_t n = 0;

spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
if (!ldata->icanon) {
n = ldata->read_cnt;
} else if (ldata->canon_data) {
n = (ldata->canon_head > ldata->read_tail) ?
ldata->canon_head - ldata->read_tail :
ldata->canon_head + (N_TTY_BUF_SIZE - ldata->read_tail);
}
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
return n;
}

Expand Down Expand Up @@ -915,19 +915,19 @@ static void eraser(unsigned char c, struct tty_struct *tty)
kill_type = WERASE;
else {
if (!L_ECHO(tty)) {
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
(N_TTY_BUF_SIZE - 1));
ldata->read_head = ldata->canon_head;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
return;
}
if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
(N_TTY_BUF_SIZE - 1));
ldata->read_head = ldata->canon_head;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
finish_erasing(ldata);
echo_char(KILL_CHAR(tty), tty);
/* Add a newline if ECHOK is on and ECHOKE is off. */
Expand Down Expand Up @@ -961,10 +961,10 @@ static void eraser(unsigned char c, struct tty_struct *tty)
break;
}
cnt = (ldata->read_head - head) & (N_TTY_BUF_SIZE-1);
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_head = head;
ldata->read_cnt -= cnt;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
if (L_ECHO(tty)) {
if (L_ECHOPRT(tty)) {
if (!ldata->erasing) {
Expand Down Expand Up @@ -1344,12 +1344,12 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
put_tty_queue(c, ldata);

handle_newline:
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
set_bit(ldata->read_head, ldata->read_flags);
put_tty_queue_nolock(c, ldata);
ldata->canon_head = ldata->read_head;
ldata->canon_data++;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
if (waitqueue_active(&tty->read_wait))
wake_up_interruptible(&tty->read_wait);
Expand Down Expand Up @@ -1423,7 +1423,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
unsigned long cpuflags;

if (ldata->real_raw) {
spin_lock_irqsave(&ldata->read_lock, cpuflags);
raw_spin_lock_irqsave(&ldata->read_lock, cpuflags);
i = min(N_TTY_BUF_SIZE - ldata->read_cnt,
N_TTY_BUF_SIZE - ldata->read_head);
i = min(count, i);
Expand All @@ -1439,7 +1439,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
memcpy(ldata->read_buf + ldata->read_head, cp, i);
ldata->read_head = (ldata->read_head + i) & (N_TTY_BUF_SIZE-1);
ldata->read_cnt += i;
spin_unlock_irqrestore(&ldata->read_lock, cpuflags);
raw_spin_unlock_irqrestore(&ldata->read_lock, cpuflags);
} else {
for (i = count, p = cp, f = fp; i; i--, p++) {
if (f)
Expand Down Expand Up @@ -1635,7 +1635,7 @@ static int n_tty_open(struct tty_struct *tty)
mutex_init(&ldata->atomic_read_lock);
mutex_init(&ldata->output_lock);
mutex_init(&ldata->echo_lock);
spin_lock_init(&ldata->read_lock);
raw_spin_lock_init(&ldata->read_lock);

/* These are ugly. Currently a malloc failure here can panic */
ldata->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
Expand Down Expand Up @@ -1703,24 +1703,24 @@ static int copy_from_read_buf(struct tty_struct *tty,
bool is_eof;

retval = 0;
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
n = min(ldata->read_cnt, N_TTY_BUF_SIZE - ldata->read_tail);
n = min(*nr, n);
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
if (n) {
retval = copy_to_user(*b, &ldata->read_buf[ldata->read_tail], n);
n -= retval;
is_eof = n == 1 &&
ldata->read_buf[ldata->read_tail] == EOF_CHAR(tty);
tty_audit_add_data(tty, &ldata->read_buf[ldata->read_tail], n,
ldata->icanon);
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
ldata->read_tail = (ldata->read_tail + n) & (N_TTY_BUF_SIZE-1);
ldata->read_cnt -= n;
/* Turn single EOF into zero-length read */
if (L_EXTPROC(tty) && ldata->icanon && is_eof && !ldata->read_cnt)
n = 0;
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
*b += n;
*nr -= n;
}
Expand Down Expand Up @@ -1900,7 +1900,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,

if (ldata->icanon && !L_EXTPROC(tty)) {
/* N.B. avoid overrun if nr == 0 */
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
while (nr && ldata->read_cnt) {
int eol;

Expand All @@ -1918,25 +1918,25 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
if (--ldata->canon_data < 0)
ldata->canon_data = 0;
}
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);

if (!eol || (c != __DISABLED_CHAR)) {
if (tty_put_user(tty, c, b++)) {
retval = -EFAULT;
b--;
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
break;
}
nr--;
}
if (eol) {
tty_audit_push(tty);
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
break;
}
spin_lock_irqsave(&ldata->read_lock, flags);
raw_spin_lock_irqsave(&ldata->read_lock, flags);
}
spin_unlock_irqrestore(&ldata->read_lock, flags);
raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
if (retval)
break;
} else {
Expand Down

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