speakup ttyio: Do not schedule() in ttyio_in_nowait

With the ltlk and spkout drivers, the index read function, i.e.
in_nowait, is getting called from the read_all_doc mechanism, from
the timer softirq:

Call Trace:
 <IRQ>
 dump_stack+0x71/0x98
 dequeue_task_idle+0x1f/0x28
 __schedule+0x167/0x5d6
 ? trace_hardirqs_on+0x2e/0x3a
 ? usleep_range+0x7f/0x7f
 schedule+0x8a/0xae
 schedule_timeout+0xb1/0xea
 ? del_timer_sync+0x31/0x31
 do_wait_for_common+0xba/0x12b
 ? wake_up_q+0x45/0x45
 wait_for_common+0x37/0x50
 ttyio_in+0x2a/0x6b
 spk_ttyio_in_nowait+0xc/0x13
 spk_get_index_count+0x20/0x93
 cursor_done+0x1c6/0x4c6
 ? read_all_doc+0xb1/0xb1
 call_timer_fn+0x89/0x140
 run_timer_softirq+0x164/0x1a5
 ? read_all_doc+0xb1/0xb1
 ? hrtimer_forward+0x7b/0x87
 ? timerqueue_add+0x62/0x68
 ? enqueue_hrtimer+0x95/0x9f
 __do_softirq+0x181/0x31f
 irq_exit+0x6a/0x86
smp_apic_timer_interrupt+0x15e/0x183
 apic_timer_interrupt+0xf/0x20
 </IRQ>

We thus should not schedule() at all, even with timeout == 0, this
crashes the kernel.  We can however use try_wait_for_completion()
instead of wait_for_completion_timeout(0).

Cc: stable@vger.kernel.org
Reported-by: John Covici <covici@ccs.covici.com>
Tested-by: John Covici <covici@ccs.covici.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20201108131233.tadycr73sxlvodgo@functionSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/accessibility/speakup/spk_ttyio.c

@@ -298,11 +298,13 @@ static unsigned char ttyio_in(int timeout)
 	struct spk_ldisc_data *ldisc_data = speakup_tty->disc_data;
 	char rv;
 
-	if (wait_for_completion_timeout(&ldisc_data->completion,
+	if (!timeout) {
+		if (!try_wait_for_completion(&ldisc_data->completion))
+			return 0xff;
+	} else if (wait_for_completion_timeout(&ldisc_data->completion,
 					usecs_to_jiffies(timeout)) == 0) {
-		if (timeout)
-			pr_warn("spk_ttyio: timeout (%d)  while waiting for input\n",
-				timeout);
+		pr_warn("spk_ttyio: timeout (%d)  while waiting for input\n",
+			timeout);
 		return 0xff;
 	}