um: Rewrite the sigio workaround based on epoll and tgkill

The existing sigio workaround implementation removes FDs from the
poll when events are triggered, requiring users to re-add them via
add_sigio_fd() after processing. This introduces a potential race
condition between FD removal in write_sigio_thread() and next_poll
update in __add_sigio_fd(), and is inefficient due to frequent FD
removal and re-addition. Rewrite the implementation based on epoll
and tgkill for improved efficiency and reliability.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250315161910.4082396-2-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/drivers/random.c

@@ -79,7 +79,7 @@ static int __init rng_init (void)
 	if (err < 0)
 		goto err_out_cleanup_hw;
 
-	sigio_broken(random_fd);
+	sigio_broken();
 	hwrng.name = RNG_MODULE_NAME;
 	hwrng.read = rng_dev_read;
 

arch/um/drivers/rtc_user.c

@@ -39,7 +39,7 @@ int uml_rtc_start(bool timetravel)
 		}
 
 		/* apparently timerfd won't send SIGIO, use workaround */
-		sigio_broken(uml_rtc_irq_fds[0]);
+		sigio_broken();
 		err = add_sigio_fd(uml_rtc_irq_fds[0]);
 		if (err < 0) {
 			close(uml_rtc_irq_fds[0]);

arch/um/include/shared/os.h

@@ -314,7 +314,7 @@ extern void um_irqs_resume(void);
 extern int add_sigio_fd(int fd);
 extern int ignore_sigio_fd(int fd);
 extern void maybe_sigio_broken(int fd);
-extern void sigio_broken(int fd);
+extern void sigio_broken(void);
 /*
  * unlocked versions for IRQ controller code.
  *

arch/um/include/shared/sigio.h

@@ -6,7 +6,6 @@
 #ifndef __SIGIO_H__
 #define __SIGIO_H__
 
-extern int write_sigio_irq(int fd);
 extern void sigio_lock(void);
 extern void sigio_unlock(void);
 

arch/um/kernel/sigio.c

@@ -8,32 +8,6 @@
 #include <os.h>
 #include <sigio.h>
 
-/* Protected by sigio_lock() called from write_sigio_workaround */
-static int sigio_irq_fd = -1;
-
-static irqreturn_t sigio_interrupt(int irq, void *data)
-{
-	char c;
-
-	os_read_file(sigio_irq_fd, &c, sizeof(c));
-	return IRQ_HANDLED;
-}
-
-int write_sigio_irq(int fd)
-{
-	int err;
-
-	err = um_request_irq(SIGIO_WRITE_IRQ, fd, IRQ_READ, sigio_interrupt,
-			     0, "write sigio", NULL);
-	if (err < 0) {
-		printk(KERN_ERR "write_sigio_irq : um_request_irq failed, "
-		       "err = %d\n", err);
-		return -1;
-	}
-	sigio_irq_fd = fd;
-	return 0;
-}
-
 /* These are called from os-Linux/sigio.c to protect its pollfds arrays. */
 static DEFINE_MUTEX(sigio_mutex);