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

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[ Upstream commit 887c5c12e80c8424bd471122d2e8b6b462e12874 ]

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@intel.com>
Link: https://patch.msgid.link/20250314130815.226872-1-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Benjamin Berg
2025-03-14 14:08:15 +01:00
committed by Greg Kroah-Hartman
parent 0a205fdbb3
commit da780c4a07
2 changed files with 13 additions and 0 deletions
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2
arch/um/include/linux/time-internal.h
View File

@@ -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.

11
arch/um/kernel/skas/syscall.c
View File

@@ -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);