linux-kernel - Re: [tip:locking/core 6/10] arch/x86/include/asm/rmwcc.h:23:17: error: jump into statement expression

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Date:   Mon, 22 Oct 2018 12:08:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kbuild test robot <lkp@...el.com>
Cc:     kbuild-all@...org, linux-kernel@...r.kernel.org,
        tipbuild@...or.com, Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [tip:locking/core 6/10] arch/x86/include/asm/rmwcc.h:23:17:
 error: jump into statement expression

On Sat, Oct 20, 2018 at 11:05:11AM +0800, kbuild test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking/core
> head:   01a14bda11add9dcd4a59200f13834d634559935
> commit: 7aa54be2976550f17c11a1c3e3630002dea39303 [6/10] locking/qspinlock, x86: Provide liveness guarantee
> config: x86_64-randconfig-u0-10201040 (attached as .config)
> compiler: gcc-5 (Debian 5.5.0-3) 5.4.1 20171010
> reproduce:
>         git checkout 7aa54be2976550f17c11a1c3e3630002dea39303
>         # save the attached .config to linux build tree
>         make ARCH=x86_64 
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from arch/x86/include/asm/atomic.h:5:0,
>                     from include/linux/atomic.h:7,
>                     from include/linux/crypto.h:20,
>                     from arch/x86/kernel/asm-offsets.c:9:
>    arch/x86/include/asm/qspinlock.h: In function 'queued_fetch_set_pending_acquire':
> >> arch/x86/include/asm/rmwcc.h:23:17: error: jump into statement expression
>        : clobbers : cc_label);    \
>                     ^
>    include/linux/compiler.h:58:42: note: in definition of macro '__trace_if'
>      if (__builtin_constant_p(!!(cond)) ? !!(cond) :   \
>                                              ^

ARGH; so, this:

#define __GEN_RMWcc(fullop, _var, cc, clobbers, ...)			\
({									\
	bool c = false;							\
	asm_volatile_goto (fullop "; j" #cc " %l[cc_label]"		\
			: : [var] "m" (_var), ## __VA_ARGS__		\
			: clobbers : cc_label);				\
	if (0) {							\
cc_label:	c = true;						\
	}								\
	c;								\
})

static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock)
{
	u32 val = 0;

	if (GEN_BINARY_RMWcc(LOCK_PREFIX "btsl", lock->val.counter, c,
			     "I", _Q_PENDING_OFFSET))
		val |= _Q_PENDING_VAL;

	val |= atomic_read(&lock->val) & ~_Q_PENDING_MASK;

	return val;
}

fails to compile when combined with this:

#define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
#define __trace_if(cond) \
	if (__builtin_constant_p(!!(cond)) ? !!(cond) :			\
	({								\
		int ______r;						\
		static struct ftrace_branch_data			\
			__attribute__((__aligned__(4)))			\
			__attribute__((section("_ftrace_branch")))	\
			______f = {					\
				.func = __func__,			\
				.file = __FILE__,			\
				.line = __LINE__,			\
			};						\
		______r = !!(cond);					\
		______f.miss_hit[______r]++;				\
		______r;						\
	}))

Because that moves the __GEN_RMWcc into a statement expression and GCC
apparently doesn't like labels inside statement expressions.

If we avoid if() and rewrite queued_fetch_set_pending_acquire() like so:

static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lock)
{
	bool pending;
	u32 val;

	pending = GEN_BINARY_RMWcc(LOCK_PREFIX "btsl", lock->val.counter, c,
				   "I", _Q_PENDING_OFFSET);

	val = pending * _Q_PENDING_VAL;
	val |= atomic_read(&lock->val) & ~_Q_PENDING_MASK;

	return val;
}

then it compiles again; but urgh.

Anybody see a better solution?