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A number of situations currently require the heavyweight smp_mb(), even though there is no need to order prior stores against later loads. Many architectures have much cheaper ways to handle these situations, but the Linux kernel currently has no portable way to make use of them. This commit therefore supplies smp_load_acquire() and smp_store_release() to remedy this situation. The new smp_load_acquire() primitive orders the specified load against any subsequent reads or writes, while the new smp_store_release() primitive orders the specifed store against any prior reads or writes. These primitives allow array-based circular FIFOs to be implemented without an smp_mb(), and also allow a theoretical hole in rcu_assign_pointer() to be closed at no additional expense on most architectures. In addition, the RCU experience transitioning from explicit smp_read_barrier_depends() and smp_wmb() to rcu_dereference() and rcu_assign_pointer(), respectively resulted in substantial improvements in readability. It therefore seems likely that replacing other explicit barriers with smp_load_acquire() and smp_store_release() will provide similar benefits. It appears that roughly half of the explicit barriers in core kernel code might be so replaced. [Changelog by PaulMck] Reviewed-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Will Deacon <will.deacon@arm.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Michael Ellerman <michael@ellerman.id.au> Cc: Michael Neuling <mikey@neuling.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Victor Kaplansky <VICTORK@il.ibm.com> Cc: Tony Luck <tony.luck@intel.com> Cc: Oleg Nesterov <oleg@redhat.com> Link: http://lkml.kernel.org/r/20131213150640.908486364@infradead.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
103 lines
2.7 KiB
C
103 lines
2.7 KiB
C
/*
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* Based on arch/arm/include/asm/barrier.h
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*
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* Copyright (C) 2012 ARM Ltd.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef __ASM_BARRIER_H
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#define __ASM_BARRIER_H
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#ifndef __ASSEMBLY__
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#define sev() asm volatile("sev" : : : "memory")
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#define wfe() asm volatile("wfe" : : : "memory")
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#define wfi() asm volatile("wfi" : : : "memory")
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#define isb() asm volatile("isb" : : : "memory")
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#define dsb() asm volatile("dsb sy" : : : "memory")
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#define mb() dsb()
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#define rmb() asm volatile("dsb ld" : : : "memory")
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#define wmb() asm volatile("dsb st" : : : "memory")
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#ifndef CONFIG_SMP
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#define smp_mb() barrier()
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#define smp_rmb() barrier()
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#define smp_wmb() barrier()
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#define smp_store_release(p, v) \
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do { \
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compiletime_assert_atomic_type(*p); \
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smp_mb(); \
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ACCESS_ONCE(*p) = (v); \
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} while (0)
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#define smp_load_acquire(p) \
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({ \
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typeof(*p) ___p1 = ACCESS_ONCE(*p); \
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compiletime_assert_atomic_type(*p); \
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smp_mb(); \
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___p1; \
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})
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#else
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#define smp_mb() asm volatile("dmb ish" : : : "memory")
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#define smp_rmb() asm volatile("dmb ishld" : : : "memory")
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#define smp_wmb() asm volatile("dmb ishst" : : : "memory")
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#define smp_store_release(p, v) \
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do { \
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compiletime_assert_atomic_type(*p); \
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switch (sizeof(*p)) { \
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case 4: \
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asm volatile ("stlr %w1, %0" \
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: "=Q" (*p) : "r" (v) : "memory"); \
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break; \
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case 8: \
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asm volatile ("stlr %1, %0" \
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: "=Q" (*p) : "r" (v) : "memory"); \
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break; \
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} \
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} while (0)
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#define smp_load_acquire(p) \
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({ \
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typeof(*p) ___p1; \
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compiletime_assert_atomic_type(*p); \
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switch (sizeof(*p)) { \
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case 4: \
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asm volatile ("ldar %w0, %1" \
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: "=r" (___p1) : "Q" (*p) : "memory"); \
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break; \
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case 8: \
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asm volatile ("ldar %0, %1" \
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: "=r" (___p1) : "Q" (*p) : "memory"); \
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break; \
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} \
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___p1; \
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})
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#endif
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#define read_barrier_depends() do { } while(0)
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#define smp_read_barrier_depends() do { } while(0)
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#define set_mb(var, value) do { var = value; smp_mb(); } while (0)
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#define nop() asm volatile("nop");
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#endif /* __ASSEMBLY__ */
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#endif /* __ASM_BARRIER_H */
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