Commit 381d2f95 authored by Johannes Berg's avatar Johannes Berg Committed by Richard Weinberger
Browse files

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: default avatarJohannes Berg <johannes.berg@intel.com>
Signed-off-by: default avatarRichard Weinberger <richard@nod.at>
parent ae0dc67c

arch/um/kernel/skas/syscall.c

+20 −14
Original line number Diff line number Diff line
@@ -12,23 +12,13 @@ 
#include <sysdep/syscalls.h>

#include <linux/time-internal.h>

#include <asm/unistd.h>

#include <asm/delay.h>



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);