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Commit f862eefe authored by Chris Metcalf's avatar Chris Metcalf
Browse files

tile: use a more conservative __my_cpu_offset in CONFIG_PREEMPT 



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 509eb76e.

Cc: stable@vger.kernel.org
Signed-off-by: default avatarChris Metcalf <cmetcalf@tilera.com>
parent 3f725c5b

arch/tile/include/asm/percpu.h

+31 −3
Original line number Diff line number Diff line
@@ -15,9 +15,37 @@ 
#ifndef _ASM_TILE_PERCPU_H

#define _ASM_TILE_PERCPU_H



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");



#ifdef CONFIG_PREEMPT

/*

 * 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()

#else

/*

 * 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

#endif



#define set_my_cpu_offset(tp) (my_cpu_offset_reg = (tp))



#include <asm-generic/percpu.h>