sched: Remove PREEMPT_NEED_RESCHED from generic code
While hunting a preemption issue with Alexander, Ben noticed that the
currently generic PREEMPT_NEED_RESCHED stuff is horribly broken for
load-store architectures.
We currently rely on the IPI to fold TIF_NEED_RESCHED into
PREEMPT_NEED_RESCHED, but when this IPI lands while we already have
a load for the preempt-count but before the store, the store will erase
the PREEMPT_NEED_RESCHED change.
The current preempt-count only works on load-store archs because
interrupts are assumed to be completely balanced wrt their preempt_count
fiddling; the previous preempt_count load will match the preempt_count
state after the interrupt and therefore nothing gets lost.
This patch removes the PREEMPT_NEED_RESCHED usage from generic code and
pushes it into x86 arch code; the generic code goes back to relying on
TIF_NEED_RESCHED.
Boot tested on x86_64 and compile tested on ppc64.
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-and-Tested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131128132641.GP10022@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
index ddf2b42..1cd3f5d 100644
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -3,13 +3,11 @@
#include <linux/thread_info.h>
-/*
- * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users
- * that think a non-zero value indicates we cannot preempt.
- */
+#define PREEMPT_ENABLED (0)
+
static __always_inline int preempt_count(void)
{
- return current_thread_info()->preempt_count & ~PREEMPT_NEED_RESCHED;
+ return current_thread_info()->preempt_count;
}
static __always_inline int *preempt_count_ptr(void)
@@ -17,11 +15,6 @@
return ¤t_thread_info()->preempt_count;
}
-/*
- * We now loose PREEMPT_NEED_RESCHED and cause an extra reschedule; however the
- * alternative is loosing a reschedule. Better schedule too often -- also this
- * should be a very rare operation.
- */
static __always_inline void preempt_count_set(int pc)
{
*preempt_count_ptr() = pc;
@@ -41,28 +34,17 @@
task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
} while (0)
-/*
- * We fold the NEED_RESCHED bit into the preempt count such that
- * preempt_enable() can decrement and test for needing to reschedule with a
- * single instruction.
- *
- * We invert the actual bit, so that when the decrement hits 0 we know we both
- * need to resched (the bit is cleared) and can resched (no preempt count).
- */
-
static __always_inline void set_preempt_need_resched(void)
{
- *preempt_count_ptr() &= ~PREEMPT_NEED_RESCHED;
}
static __always_inline void clear_preempt_need_resched(void)
{
- *preempt_count_ptr() |= PREEMPT_NEED_RESCHED;
}
static __always_inline bool test_preempt_need_resched(void)
{
- return !(*preempt_count_ptr() & PREEMPT_NEED_RESCHED);
+ return false;
}
/*
@@ -81,7 +63,12 @@
static __always_inline bool __preempt_count_dec_and_test(void)
{
- return !--*preempt_count_ptr();
+ /*
+ * Because of load-store architectures cannot do per-cpu atomic
+ * operations; we cannot use PREEMPT_NEED_RESCHED because it might get
+ * lost.
+ */
+ return !--*preempt_count_ptr() && tif_need_resched();
}
/*
@@ -89,7 +76,7 @@
*/
static __always_inline bool should_resched(void)
{
- return unlikely(!*preempt_count_ptr());
+ return unlikely(!preempt_count() && tif_need_resched());
}
#ifdef CONFIG_PREEMPT