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SubjectRe: [PATCH] vt: Fix sleeping functions called from atomic context
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On 2021/11/17 17:54, Greg Kroah-Hartman wrote:
> Great, you have a reproducer, so you should be able to duplicate this
> locally to figure out what is really happening here.

Until commit ac751efa6a0d70f2 ("console: rename acquire/release_console_sem() to
console_lock/unlock()"), do_con_write() was surely designed to be able to sleep.

$ git blame ac751efa6a0d7~1 drivers/tty/vt/vt.c
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2122) /* acquires console_sem */
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2123) static int do_con_write(struct tty_struct *tty, const unsigned char *buf, int count)
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2124) {
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2125) #ifdef VT_BUF_VRAM_ONLY
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2126) #define FLUSH do { } while(0);
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2127) #else
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2128) #define FLUSH if (draw_x >= 0) { \
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2129) vc->vc_sw->con_putcs(vc, (u16 *)draw_from, (u16 *)draw_to - (u16 *)draw_from, vc->vc_y, draw_x); \
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2130) draw_x = -1; \
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2131) }
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2132) #endif
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2133)
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2134) int c, tc, ok, n = 0, draw_x = -1;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2135) unsigned int currcons;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2136) unsigned long draw_from = 0, draw_to = 0;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2137) struct vc_data *vc;
2f1a2ccb9c0de drivers/char/vt.c (Egmont Koblinger 2007-05-08 00:30:37 -0700 2138) unsigned char vc_attr;
0341a4d0fdd2a drivers/char/vt.c (Karl Dahlke 2008-04-28 02:14:25 -0700 2139) struct vt_notifier_param param;
2f1a2ccb9c0de drivers/char/vt.c (Egmont Koblinger 2007-05-08 00:30:37 -0700 2140) uint8_t rescan;
2f1a2ccb9c0de drivers/char/vt.c (Egmont Koblinger 2007-05-08 00:30:37 -0700 2141) uint8_t inverse;
2f1a2ccb9c0de drivers/char/vt.c (Egmont Koblinger 2007-05-08 00:30:37 -0700 2142) uint8_t width;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2143) u16 himask, charmask;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2144)
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2145) if (in_interrupt())
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2146) return count;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2147)
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2148) might_sleep();
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2149)
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2150) acquire_console_sem();
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2151) vc = tty->driver_data;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2152) if (vc == NULL) {
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2153) printk(KERN_ERR "vt: argh, driver_data is NULL !\n");
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2154) release_console_sem();
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2155) return 0;
^1da177e4c3f4 drivers/char/vt.c (Linus Torvalds 2005-04-16 15:20:36 -0700 2156) }

Until that commit, n_hdlc_send_frames() was prepared for being interrupted by signal
while sleeping.

$ git blame ac751efa6a0d7~1 drivers/tty/n_hdlc.c
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 379) /**
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 380) * n_hdlc_send_frames - send frames on pending send buffer list
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 381) * @n_hdlc - pointer to ldisc instance data
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 382) * @tty - pointer to tty instance data
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 383) *
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 384) * Send frames on pending send buffer list until the driver does not accept a
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 385) * frame (busy) this function is called after adding a frame to the send buffer
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 386) * list and by the tty wakeup callback.
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 387) */
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 388) static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 389) {
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 390) register int actual;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 391) unsigned long flags;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 392) struct n_hdlc_buf *tbuf;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 393)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 394) if (debuglevel >= DEBUG_LEVEL_INFO)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 395) printk("%s(%d)n_hdlc_send_frames() called\n",__FILE__,__LINE__);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 396) check_again:
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 397)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 398) spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 399) if (n_hdlc->tbusy) {
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 400) n_hdlc->woke_up = 1;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 401) spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 402) return;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 403) }
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 404) n_hdlc->tbusy = 1;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 405) n_hdlc->woke_up = 0;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 406) spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 407)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 408) /* get current transmit buffer or get new transmit */
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 409) /* buffer from list of pending transmit buffers */
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 410)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 411) tbuf = n_hdlc->tbuf;
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 412) if (!tbuf)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 413) tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 414)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 415) while (tbuf) {
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 416) if (debuglevel >= DEBUG_LEVEL_INFO)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 417) printk("%s(%d)sending frame %p, count=%d\n",
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 418) __FILE__,__LINE__,tbuf,tbuf->count);
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 419)
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 420) /* Send the next block of data to device */
^1da177e4c3f4 drivers/char/n_hdlc.c (Linus Torvalds 2005-04-16 15:20:36 -0700 421) tty->flags |= (1 << TTY_DO_WRITE_WAKEUP);
f34d7a5b7010b drivers/char/n_hdlc.c (Alan Cox 2008-04-30 00:54:13 -0700 422) actual = tty->ops->write(tty, tbuf->buf, tbuf->count);
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 423)
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 424) /* rollback was possible and has been done */
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 425) if (actual == -ERESTARTSYS) {
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 426) n_hdlc->tbuf = tbuf;
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 427) break;
b0fed3140f57c drivers/char/n_hdlc.c (Jiri Slaby 2007-07-15 23:40:12 -0700 428) }

But as of commit c545b66c6922b002 ("tty: Serialize tcflow() with other tty flow
control changes"), start_tty() was already holding spinlock.

$ git blame c545b66c6922b002~1 drivers/tty/tty_io.c
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 965) void start_tty(struct tty_struct *tty)
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 966) {
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 967) unsigned long flags;
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 968)
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 969) spin_lock_irqsave(&tty->flow_lock, flags);
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 970) __start_tty(tty);
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 971) spin_unlock_irqrestore(&tty->flow_lock, flags);
f9e053dcfc02b drivers/tty/tty_io.c (Peter Hurley 2014-09-10 15:06:31 -0400 972) }

Actually, it is commit f9e053dcfc02b0ad ("tty: Serialize tty flow control changes
with flow_lock") that started calling tty->ops->start(tty) from atomic context.

$ git blame f9e053dcfc02b~1 drivers/tty/tty_io.c
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 959) void start_tty(struct tty_struct *tty)
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 960) {
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 961) unsigned long flags;
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 962) spin_lock_irqsave(&tty->ctrl_lock, flags);
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 963) if (!tty->stopped || tty->flow_stopped) {
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 964) spin_unlock_irqrestore(&tty->ctrl_lock, flags);
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 965) return;
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 966) }
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 967) tty->stopped = 0;
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 968) if (tty->link && tty->link->packet) {
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 969) tty->ctrl_status &= ~TIOCPKT_STOP;
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 970) tty->ctrl_status |= TIOCPKT_START;
4b19449db074e drivers/char/tty_io.c (Davide Libenzi 2009-03-31 15:24:24 -0700 971) wake_up_interruptible_poll(&tty->link->read_wait, POLLIN);
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 972) }
04f378b198da2 drivers/char/tty_io.c (Alan Cox 2008-04-30 00:53:29 -0700 973) spin_unlock_irqrestore(&tty->ctrl_lock, flags);
f34d7a5b7010b drivers/char/tty_io.c (Alan Cox 2008-04-30 00:54:13 -0700 974) if (tty->ops->start)
f34d7a5b7010b drivers/char/tty_io.c (Alan Cox 2008-04-30 00:54:13 -0700 975) (tty->ops->start)(tty);
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 976) /* If we have a running line discipline it may need kicking */
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 977) tty_wakeup(tty);
^1da177e4c3f4 drivers/char/tty_io.c (Linus Torvalds 2005-04-16 15:20:36 -0700 978) }

Therefore, I think that bisection will reach f9e053dcfc02b0ad, and I guess that
this bug was not noticed simply because little people tested n_hdlc driver.

Well, how to fix? Introduce a new flag for indicating "starting" state (like drivers/block/loop.c uses Lo_* state) ?

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