locking/xchg/alpha: Add unconditional memory barrier to cmpxchg()
Continuing along with the fight against smp_read_barrier_depends() [1] (or rather, against its improper use), add an unconditional barrier to cmpxchg. This guarantees that dependency ordering is preserved when a dependency is headed by an unsuccessful cmpxchg. As it turns out, the change could enable further simplification of LKMM as proposed in [2]. [1] https://marc.info/?l=linux-kernel&m=150884953419377&w=2 https://marc.info/?l=linux-kernel&m=150884946319353&w=2 https://marc.info/?l=linux-kernel&m=151215810824468&w=2 https://marc.info/?l=linux-kernel&m=151215816324484&w=2 [2] https://marc.info/?l=linux-kernel&m=151881978314872&w=2 Signed-off-by: Andrea Parri <parri.andrea@gmail.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Matt Turner <mattst88@gmail.com> Cc: Richard Henderson <rth@twiddle.net> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Cc: linux-alpha@vger.kernel.org Link: http://lkml.kernel.org/r/1519152356-4804-1-git-send-email-parri.andrea@gmail.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -128,10 +128,9 @@ ____xchg(, volatile void *ptr, unsigned long x, int size)
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* store NEW in MEM. Return the initial value in MEM. Success is
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* indicated by comparing RETURN with OLD.
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*
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* The memory barrier should be placed in SMP only when we actually
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* make the change. If we don't change anything (so if the returned
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* prev is equal to old) then we aren't acquiring anything new and
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* we don't need any memory barrier as far I can tell.
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* The memory barrier is placed in SMP unconditionally, in order to
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* guarantee that dependency ordering is preserved when a dependency
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* is headed by an unsuccessful operation.
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*/
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static inline unsigned long
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@ -150,8 +149,8 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old, unsigned char new)
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" or %1,%2,%2\n"
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" stq_c %2,0(%4)\n"
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" beq %2,3f\n"
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__ASM__MB
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"2:\n"
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__ASM__MB
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".subsection 2\n"
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"3: br 1b\n"
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".previous"
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@ -177,8 +176,8 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old, unsigned short new)
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" or %1,%2,%2\n"
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" stq_c %2,0(%4)\n"
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" beq %2,3f\n"
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__ASM__MB
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"2:\n"
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__ASM__MB
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".subsection 2\n"
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"3: br 1b\n"
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".previous"
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@ -200,8 +199,8 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
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" mov %4,%1\n"
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" stl_c %1,%2\n"
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" beq %1,3f\n"
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__ASM__MB
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"2:\n"
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__ASM__MB
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".subsection 2\n"
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"3: br 1b\n"
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".previous"
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@ -223,8 +222,8 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old, unsigned long new)
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" mov %4,%1\n"
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" stq_c %1,%2\n"
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" beq %1,3f\n"
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__ASM__MB
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"2:\n"
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__ASM__MB
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".subsection 2\n"
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"3: br 1b\n"
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".previous"
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