Missing patch in 6.12.27 - breaks UM target builds

[Christian Lamparter]

Hi,

On 3/14/25 2:08 PM, Benjamin Berg wrote:
> From: Benjamin Berg <benjamin.berg at intel.com>
>     um: work around sched_yield not yielding in time-travel mode
>
> 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>

 From what I can tell the patch mentioned above was backported to 6.12.27 by:
<https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/arch/um?id=887c5c12e80c8424bd471122d2e8b6b462e12874>

but without the upstream
|Commit 0b8b2668f9981c1fefc2ef892bd915288ef01f33
|Author: Benjamin Berg <benjamin.berg at intel.com>
|Date:   Thu Oct 10 16:25:37 2024 +0200
|  um: insert scheduler ticks when userspace does not yield
|
|   In time-travel mode userspace can do a lot of work without any time
|   passing. Unfortunately, this can result in OOM situations as the RCU
|  core code will never be run. [...]

the kernel build for 6.12.27 for the UM-Target will fail:

| /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

is it possible to backport 0b8b2668f9981c1fefc2ef892bd915288ef01f33 too?
Or is it better to revert 887c5c12e80c8424bd471122d2e8b6b462e12874 again
in the stable releases?

Best Regards,
Christian Lamparter

>
> ---
>
> 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);
>