[PATCH 6.12] Revert "um: work around sched_yield not yielding in time-travel mode"

[Christian Lamparter <chunkeey@gmail.com>]

This reverts commit 887c5c12e80c ("um: work around sched_yield not yielding in time-travel mode")
added with v6.12.25.

Reason being that the patch depends on at least
commit 0b8b2668f998 ("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 0b8b2668f998 ("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>

---

Just in case, this throws someone else for a space-time loop:

What's interessting/very strange strange about this time-travel stuff:
|commit 0b8b2668f998 ("um: insert scheduler ticks when userspace does not yield")

 $ git describe 0b8b2668f998
=> v6.12-rc2-43-g0b8b2668f998
(from what I know this is 43 patches on top of v6.12-rc2 as per the man page:
"The command finds the most recent tag that is reachable from a commit. [...]
it suffixes the tag name with the number of additional commits on top of the tagged
object and the abbreviated object name of the most recent commit."

But it was merged as part of: uml-for-linus-6.13-rc1 :
https://lore.kernel.org/lkml/1155823186.11802667.1732921581257.JavaMail.zimbra@nod.at/
---
 arch/um/include/linux/time-internal.h |  2 --
 arch/um/kernel/skas/syscall.c         | 11 -----------
 2 files changed, 13 deletions(-)

diff --git a/arch/um/include/linux/time-internal.h b/arch/um/include/linux/time-internal.h
index 138908b999d7..b22226634ff6 100644
--- a/arch/um/include/linux/time-internal.h
+++ b/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.
diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c
index a5beaea2967e..b09e85279d2b 100644
--- a/arch/um/kernel/skas/syscall.c
+++ b/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);
 
-- 
2.49.0