linux-kernel - Re: [PATCH 1/2 v4] cleanup: Add cond

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Date: Tue, 13 Feb 2024 17:51:26 +0100
From: "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>,
 Dan Williams <dan.j.williams@...el.com>, linux-kernel@...r.kernel.org
Cc: linux-cxl@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
 Dave Jiang <dave.jiang@...el.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 1/2 v4] cleanup: Add cond_guard() to conditional guards

On Thursday, 8 February 2024 14:04:23 CET Fabio M. De Francesco wrote:
> Add cond_guard() macro to conditional guards.
> 
> cond_guard() is a guard to be used with the conditional variants of locks,
> like down_read_trylock() or mutex_lock_interruptible().
> 
> It takes a statement (or statement-expression) that is passed as its
> second argument. That statement (or statement-expression) is executed if
> waiting for a lock is interrupted or if a _trylock() fails in case of
> contention.
> 
> Usage example:
> 
> 	cond_guard(mutex_intr, return -EINTR, &mutex);
> 
> Consistent with other usage of _guard(), locks are unlocked at the exit of
> the scope where cond_guard() is called.
> 
[snip]
> 
> +#define cond_guard(_name, _fail, args...) \
> +	CLASS(_name, scope)(args); \
> +	if (!__guard_ptr(_name)(&scope)) _fail; \
> +	else { }
> +

I have converted and tested several functions in drivers/cxl and found that 
there are cases where this macro needs to be called twice in the same scope.

The current implementation fails to compile because any subsequent call to 
cond_guard() redefines "scope".

I have a solution for this, which is to instantiate a differently named 
variable each time cond_guard() is used:

#define __UNIQUE_LINE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
#define cond_guard(_name, _fail, args...) \
        CLASS(_name, __UNIQUE_LINE_ID(scope))(args); \
        if (!__guard_ptr(_name)(&__UNIQUE_LINE_ID(scope))) _fail; \
        else { }

But, before sending v5, I think it's best to wait for comments from those with 
more experience than me.

Fabio