[Xen-devel] [PATCH] binary or instead of logical in timer sync

[Steven Rostedt <srostedt@xxxxxxxxxx>]

Hi, I found this little bug and here's a patch.

The reading of a timer is determined if 1. the hypervisor is not 
currently updating it (where it sets the LSB of the version) or 2. the 
kernel didn't finish reading it before the hypervisor updated it (the 
kernel version doesn't match the hypervisor version).
But the current code doesn't test the above case. Instead, by using a 
binary or instead of a logical one, it would only repeat if the 
hypervisor was updating  __and__ we read the version before it started 
updating (or we read it before we started updating, but the hypervisor 
finished updating between the first part of the or and the second check).

I'm not use to thunderbird so I'm sending both an attachment and an 
inline cut and paste.
-- Steve

Signed-off-by: Steven Rostedt <srostedt@xxxxxxxxxx>

diff -r 9632ececc8f4 linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
--- a/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c  Wed Aug 02 10:13:30 
2006 +0100
+++ b/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c  Wed Aug 02 22:54:09 
2006 -0400
@@ -292,7 +292,7 @@ static void update_wallclock(void)
                shadow_tv.tv_sec  = s->wc_sec;
                shadow_tv.tv_nsec = s->wc_nsec;
                rmb();
-       } while ((s->wc_version & 1) | (shadow_tv_version ^ s->wc_version));
+       } while ((s->wc_version & 1) || (shadow_tv_version ^ s->wc_version));

        if (!independent_wallclock)
                __update_wallclock(shadow_tv.tv_sec, shadow_tv.tv_nsec);
@@ -319,7 +319,7 @@ static void get_time_values_from_xen(voi
                dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
                dst->tsc_shift         = src->tsc_shift;
                rmb();
-       } while ((src->version & 1) | (dst->version ^ src->version));
+       } while ((src->version & 1) || (dst->version ^ src->version));

        dst->tsc_to_usec_mul = dst->tsc_to_nsec_mul / 1000;
}


diff -r 9632ececc8f4 linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
--- a/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c  Wed Aug 02 10:13:30 
2006 +0100
+++ b/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c  Wed Aug 02 22:54:09 
2006 -0400
@@ -292,7 +292,7 @@ static void update_wallclock(void)
                shadow_tv.tv_sec  = s->wc_sec;
                shadow_tv.tv_nsec = s->wc_nsec;
                rmb();
-       } while ((s->wc_version & 1) | (shadow_tv_version ^ s->wc_version));
+       } while ((s->wc_version & 1) || (shadow_tv_version ^ s->wc_version));
 
        if (!independent_wallclock)
                __update_wallclock(shadow_tv.tv_sec, shadow_tv.tv_nsec);
@@ -319,7 +319,7 @@ static void get_time_values_from_xen(voi
                dst->tsc_to_nsec_mul   = src->tsc_to_system_mul;
                dst->tsc_shift         = src->tsc_shift;
                rmb();
-       } while ((src->version & 1) | (dst->version ^ src->version));
+       } while ((src->version & 1) || (dst->version ^ src->version));
 
        dst->tsc_to_usec_mul = dst->tsc_to_nsec_mul / 1000;
 }

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