Missing patch in 6.12.27 - breaks UM target builds

[Benjamin Berg]

Hi,

I think it is better to just not backport
0b8b2668f9981c1fefc2ef892bd915288ef01f33 ("um: insert scheduler ticks
when userspace does not yield").

Benjamin

On Thu, 2025-05-08 at 19:00 +0200, Christian Lamparter wrote:
> 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/commi
> t/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);
> >   
> 
>