path: root/arch
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authorChris Metcalf <cmetcalf@tilera.com>2013-09-26 13:24:53 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-10-13 16:08:34 -0700
commitbc30c3576d85f7d02f5620cbe8d03f4064c6a73a (patch)
treeb9f7610b3391bc56629e74ae043df04ef1be9cea /arch
parent22c748d6172867030c228d4737fdef0e9713a125 (diff)
tile: use a more conservative __my_cpu_offset in CONFIG_PREEMPT
commit f862eefec0b68e099a9fa58d3761ffb10bad97e1 upstream. It turns out the kernel relies on barrier() to force a reload of the percpu offset value. Since we can't easily modify the definition of barrier() to include "tp" as an output register, we instead provide a definition of __my_cpu_offset as extended assembly that includes a fake stack read to hazard against barrier(), forcing gcc to know that it must reread "tp" and recompute anything based on "tp" after a barrier. This fixes observed hangs in the slub allocator when we are looping on a percpu cmpxchg_double. A similar fix for ARMv7 was made in June in change 509eb76ebf97. Signed-off-by: Chris Metcalf <cmetcalf@tilera.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'arch')
1 files changed, 31 insertions, 3 deletions
diff --git a/arch/tile/include/asm/percpu.h b/arch/tile/include/asm/percpu.h
index 63294f5a8ef..4f7ae39fa20 100644
--- a/arch/tile/include/asm/percpu.h
+++ b/arch/tile/include/asm/percpu.h
@@ -15,9 +15,37 @@
-register unsigned long __my_cpu_offset __asm__("tp");
-#define __my_cpu_offset __my_cpu_offset
-#define set_my_cpu_offset(tp) (__my_cpu_offset = (tp))
+register unsigned long my_cpu_offset_reg asm("tp");
+ * For full preemption, we can't just use the register variable
+ * directly, since we need barrier() to hazard against it, causing the
+ * compiler to reload anything computed from a previous "tp" value.
+ * But we also don't want to use volatile asm, since we'd like the
+ * compiler to be able to cache the value across multiple percpu reads.
+ * So we use a fake stack read as a hazard against barrier().
+ * The 'U' constraint is like 'm' but disallows postincrement.
+ */
+static inline unsigned long __my_cpu_offset(void)
+ unsigned long tp;
+ register unsigned long *sp asm("sp");
+ asm("move %0, tp" : "=r" (tp) : "U" (*sp));
+ return tp;
+#define __my_cpu_offset __my_cpu_offset()
+ * We don't need to hazard against barrier() since "tp" doesn't ever
+ * change with PREEMPT_NONE, and with PREEMPT_VOLUNTARY it only
+ * changes at function call points, at which we are already re-reading
+ * the value of "tp" due to "my_cpu_offset_reg" being a global variable.
+ */
+#define __my_cpu_offset my_cpu_offset_reg
+#define set_my_cpu_offset(tp) (my_cpu_offset_reg = (tp))
#include <asm-generic/percpu.h>