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

[Benjamin Berg]

From: Benjamin Berg <benjamin.berg at intel.com>

sched_yield by a userspace may not actually cause scheduling in
time-travel mode as no time has passed. In the case seen it appears to
be a badly implemented userspace spinlock in ASAN. Unfortunately, with
time-travel it causes an extreme slowdown or even deadlock depending on
the kernel configuration (CONFIG_UML_MAX_USERSPACE_ITERATIONS).

Work around it by accounting time to the process whenever it executes a
sched_yield syscall.

Signed-off-by: Benjamin Berg <benjamin.berg at intel.com>

---

I suspect it is this code in ASAN that uses sched_yield
  https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_common/sanitizer_mutex.cpp
though there are also some other places that use sched_yield.

I doubt that code is reasonable. At the same time, not sure that
sched_yield is behaving as advertised either as it obviously is not
necessarily relinquishing the CPU.
---
 arch/um/include/linux/time-internal.h |  2 ++
 arch/um/kernel/skas/syscall.c         | 11 +++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/um/include/linux/time-internal.h b/arch/um/include/linux/time-internal.h
index b22226634ff6..138908b999d7 100644
--- a/arch/um/include/linux/time-internal.h
+++ b/arch/um/include/linux/time-internal.h
@@ -83,6 +83,8 @@ 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 b09e85279d2b..a5beaea2967e 100644
--- a/arch/um/kernel/skas/syscall.c
+++ b/arch/um/kernel/skas/syscall.c
@@ -31,6 +31,17 @@ 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);
 
-- 
2.48.1