um: Make line/tty semantics use true write IRQ

BugLink: https://bugs.launchpad.net/bugs/1854855

[ Upstream commit 917e2fd2 ]

This fixes a long standing bug where large amounts of output
could freeze the tty (most commonly seen on stdio console).
While the bug has always been there it became more pronounced
after moving to the new interrupt controller.

The line semantics are now changed to have true IRQ write
semantics which should further improve the tty/line subsystem
stability and performance
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/um/drivers/line.c

@@ -260,7 +260,7 @@ static irqreturn_t line_write_interrupt(int irq, void *data)
 	if (err == 0) {
 		spin_unlock(&line->lock);
 		return IRQ_NONE;
-	} else if (err < 0) {
+	} else if ((err < 0) && (err != -EAGAIN)) {
 		line->head = line->buffer;
 		line->tail = line->buffer;
 	}