Revert "um: work around sched_yield not yielding in time-travel mode"

This reverts commit da780c4a075ba2deb05ae29f0af4a990578c7901 which is
commit 887c5c12e80c8424bd471122d2e8b6b462e12874 upstream.

Reason being that the patch depends on at least commit 0b8b266
("um: insert scheduler ticks when userspace does not yield") in order to
build. Otherwise it fails with:

| /usr/bin/ld: arch/um/kernel/skas/syscall.o: in function `handle_syscall':
|      linux-6.12.27/arch/um/kernel/skas/syscall.c:43:(.text+0xa2): undefined
| reference to `tt_extra_sched_jiffies'
| collect2: error: ld returned 1 exit status

The author Benjamin Berg commented: "I think it is better to just not
backport commit 0b8b266 ("um: insert scheduler ticks when userspace
does not yield")"

Link: https://lore.kernel.org/linux-um/8ce0b6056a9726e540f61bce77311278654219eb.camel@sipsolutions.net/
Cc: <stable@vger.kernel.org> # 6.12.y
Cc: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 6aac2c54762c4b0b1cbd8c7ffa365283e171bf5d)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>

Author: chunkeey

arch/um/include/linux/time-internal.h

@@ -83,8 +83,6 @@ extern void time_travel_not_configured(void);
 #define time_travel_del_event(...) time_travel_not_configured()
 #endif /* CONFIG_UML_TIME_TRAVEL_SUPPORT */
 
-extern unsigned long tt_extra_sched_jiffies;
-
 /*
  * Without CONFIG_UML_TIME_TRAVEL_SUPPORT this is a linker error if used,
  * which is intentional since we really shouldn't link it in that case.

arch/um/kernel/skas/syscall.c

@@ -31,17 +31,6 @@ void handle_syscall(struct uml_pt_regs *r)
 		goto out;
 
 	syscall = UPT_SYSCALL_NR(r);
-
-	/*
-	 * If no time passes, then sched_yield may not actually yield, causing
-	 * broken spinlock implementations in userspace (ASAN) to hang for long
-	 * periods of time.
-	 */
-	if ((time_travel_mode == TT_MODE_INFCPU ||
-	     time_travel_mode == TT_MODE_EXTERNAL) &&
-	    syscall == __NR_sched_yield)
-		tt_extra_sched_jiffies += 1;
-
 	if (syscall >= 0 && syscall < __NR_syscalls) {
 		unsigned long ret = EXECUTE_SYSCALL(syscall, regs);