On Monday, January 03, 2011 7:16:38 pm Colin Percival wrote:
> Author: cperciva
> Date: Tue Jan 4 00:16:38 2011
> New Revision: 216944
> URL: http://svn.freebsd.org/changeset/base/216944
>
> Log:
> Adjust the critical section protecting _xen_flush_queue to cover the
> entire range where the page mapping request queue needs to be atomically
> examined and modified.
>
> Oddly, while this doesn't seem to affect the overall rate of panics
> (running 'make index' on EC2 t1.micro instances, there are 0.6 +/- 0.1
> panics per hour, both before and after this change), it eliminates
> vm_fault from panic backtraces, leaving only backtraces going through
> vmspace_fork.
>
> Modified:
> head/sys/i386/xen/xen_machdep.c
>
> Modified: head/sys/i386/xen/xen_machdep.c
>
==============================================================================
> --- head/sys/i386/xen/xen_machdep.c Tue Jan 4 00:11:09 2011
> (r216943)
> +++ head/sys/i386/xen/xen_machdep.c Tue Jan 4 00:16:38 2011
> (r216944)
> @@ -42,6 +42,7 @@ __FBSDID("$FreeBSD$");
> #include <sys/malloc.h>
> #include <sys/mutex.h>
> #include <sys/kernel.h>
> +#include <sys/proc.h>
> #include <sys/reboot.h>
> #include <sys/sysproto.h>
>
> @@ -249,10 +250,13 @@ _xen_flush_queue(void)
> SET_VCPU();
> int _xpq_idx = XPQ_IDX;
> int error, i;
> - /* window of vulnerability here? */
>
> +#ifdef INVARIANTS
> if (__predict_true(gdtset))
> - critical_enter();
> + KASSERT(curthread->td_critnest > 0,
> + ("xen queue flush should be in a critical section"));
> +#endif
> +
You can use CRITICAL_ASSERT(curthread) instead perhaps.
> XPQ_IDX = 0;
> /* Make sure index is cleared first to avoid double updates. */
> error = HYPERVISOR_mmu_update((mmu_update_t *)&XPQ_QUEUE,
> @@ -286,8 +290,6 @@ _xen_flush_queue(void)
> }
> }
> #endif
> - if (__predict_true(gdtset))
> - critical_exit();
> if (__predict_false(error < 0)) {
> for (i = 0; i < _xpq_idx; i++)
> printf("val: %llx ptr: %llx\n",
> @@ -301,7 +303,12 @@ void
> xen_flush_queue(void)
> {
> SET_VCPU();
> +
> + if (__predict_true(gdtset))
> + critical_enter();
> if (XPQ_IDX != 0) _xen_flush_queue();
> + if (__predict_true(gdtset))
> + critical_exit();
> }
>
> static __inline void
>
--
John Baldwin
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