From 381d2f95c8aa575d5d42bf1fe0ea9a70c4bec0cf Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Tue, 27 Aug 2024 16:05:01 +0200 Subject: [PATCH] um: fix time-travel syscall scheduling hack The schedule() call there really never did anything at least since the introduction of the EEVDF scheduler, but now I found a case where we permanently hang in a loop of -ERESTARTNOINTR (due to locking.) Work around it by making any syscalls with error return take time (and then schedule after) so we cannot hang in such a loop forever. Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger --- arch/um/kernel/skas/syscall.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c index 9ee19e566da3..b09e85279d2b 100644 --- a/arch/um/kernel/skas/syscall.c +++ b/arch/um/kernel/skas/syscall.c @@ -12,23 +12,13 @@ #include #include #include +#include void handle_syscall(struct uml_pt_regs *r) { struct pt_regs *regs = container_of(r, struct pt_regs, regs); int syscall; - /* - * If we have infinite CPU resources, then make every syscall also a - * preemption point, since we don't have any other preemption in this - * case, and kernel threads would basically never run until userspace - * went to sleep, even if said userspace interacts with the kernel in - * various ways. - */ - if (time_travel_mode == TT_MODE_INFCPU || - time_travel_mode == TT_MODE_EXTERNAL) - schedule(); - /* Initialize the syscall number and default return value. */ UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp); PT_REGS_SET_SYSCALL_RETURN(regs, -ENOSYS); @@ -41,9 +31,25 @@ void handle_syscall(struct uml_pt_regs *r) goto out; syscall = UPT_SYSCALL_NR(r); - if (syscall >= 0 && syscall < __NR_syscalls) - PT_REGS_SET_SYSCALL_RETURN(regs, - EXECUTE_SYSCALL(syscall, regs)); + if (syscall >= 0 && syscall < __NR_syscalls) { + unsigned long ret = EXECUTE_SYSCALL(syscall, regs); + + PT_REGS_SET_SYSCALL_RETURN(regs, ret); + + /* + * An error value here can be some form of -ERESTARTSYS + * and then we'd just loop. Make any error syscalls take + * some time, so that it won't just loop if something is + * not ready, and hopefully other things will make some + * progress. + */ + if (IS_ERR_VALUE(ret) && + (time_travel_mode == TT_MODE_INFCPU || + time_travel_mode == TT_MODE_EXTERNAL)) { + um_udelay(1); + schedule(); + } + } out: syscall_trace_leave(regs);