Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-09-27 12:59, Mathieu Desnoyers wrote:
> On 2024-09-27 06:28, Boqun Feng wrote:
[...]
>> I replaced ADDRESS_EQ(a, b) with ADDRESS_EQ(b, a), and the compile
>> result shows it can prevent the issue:
> 
> I see, yes. It prevents the issue by making the compiler create
> a copy of the value "modified" by the asm before doing the equality
> comparison.
> 
> This means the compiler cannot derive the value for b from the first
> load when b is used after after the equality comparison.
> 
> The only downside of OPTIMIZER_HIDE_VAR() is that it adds an extra
> "mov" instruction to move the content across registers. I don't think
> it matters performance wise though, so that solution is appealing
> because it is arch-agnostic.
> 
> One small improvement over your proposed solution would be to apply
> OPTIMIZER_HIDE_VAR() on both inputs. Because this is not a volatile
> asm, it is simply optimized away if var1 or var2 is unused following
> the equality comparison. It is more convenient to prevent replacement
> of both addresses being compared by the other rather than providing
> the guarantee only on a single parameter:

Actually, your approach is better (only preserving the address
dependency on the first parameter), because it allows the second
parameter to be a constant.

Here is a diff. Please let me know if I need to improve anything wrt
comments or implementation:

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 2df665fa2964..52434eccd715 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -186,6 +186,32 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  	__asm__ ("" : "=r" (var) : "0" (var))
  #endif
  
+/*
+ * Compare an address with an expression while preserving the address
+ * dependencies for later use of the address. It should be used when
+ * comparing an address returned by rcu_dereference() with another
+ * address (either constant or in registers).
+ *
+ * This is needed to prevent the compiler SSA GVN optimization pass from
+ * replacing the register holding @addr by @expr (either a constant or a
+ * register) based on their equality, which does not preserve address
+ * dependencies and allows the following misordering speculations:
+ *
+ * - If @expr is a constant, the compiler can issue the loads which depend
+ *   on @addr before the load of @addr.
+ * - If @expr is a register populated by a prior load, weakly-ordered
+ *   CPUs can speculate loads which depend on @addr before the load of the
+ *   address they depend on.
+ */
+#ifndef ADDRESS_EQ
+#define ADDRESS_EQ(addr, expr)					\
+	({							\
+		bool __res = (addr) == (expr);			\
+		OPTIMIZER_HIDE_VAR(addr);			\
+		__res;						\
+	})
+#endif
+
  #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
  
  /**

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com