linux-kernel - Re: [PATCH v11 01/19] dyndbg: add _DPRINTK_FLAGS

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Date:   Fri, 14 Jan 2022 12:57:18 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Jim Cromie <jim.cromie@...il.com>
CC:     "jbaron@...mai.com" <jbaron@...mai.com>,
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Subject: Re: [PATCH v11 01/19] dyndbg: add _DPRINTK_FLAGS_ENABLED

On Fri, Jan 07, 2022 at 06:29:24AM +0100, Jim Cromie wrote:
>  #ifdef CONFIG_JUMP_LABEL
> -			if (dp->flags & _DPRINTK_FLAGS_PRINT) {
> -				if (!(modifiers->flags & _DPRINTK_FLAGS_PRINT))
> +			if (dp->flags & _DPRINTK_FLAGS_ENABLED) {
> +				if (!(modifiers->flags & _DPRINTK_FLAGS_ENABLED))
>  					static_branch_disable(&dp->key.dd_key_true);
> -			} else if (modifiers->flags & _DPRINTK_FLAGS_PRINT)
> +			} else if (modifiers->flags & _DPRINTK_FLAGS_ENABLED)
>  				static_branch_enable(&dp->key.dd_key_true);
>  #endif
>  			dp->flags = newflags;
> -- 
> 2.33.1
> 

I haven't tested it so I could be mistaken, but when
_DPRINTK_FLAGS_ENABLED gets two flags in the next patch, it looks like
this code still has the problem which I mentioned in
https://lore.kernel.org/lkml/20211209150910.GA23668@axis.com/?

| I noticed a bug inside the CONFIG_JUMP_LABEL handling (also present
| in the last version I posted) which should be fixed as part of the
| diff below (I've added a comment).
| [...] 
|  #ifdef CONFIG_JUMP_LABEL
| -			if (dp->flags & _DPRINTK_FLAGS_PRINT) {
| -				if (!(modifiers->flags & _DPRINTK_FLAGS_PRINT))
| +			if (dp->flags & _DPRINTK_FLAGS_ENABLE) {
| +				/*
| +				 * The newflags check is to ensure that the
| +				 * static branch doesn't get disabled in step
| +				 * 3:
| +				 *
| +				 * (1) +pf
| +				 * (2) +x
| +				 * (3) -pf
| +				 */
| +				if (!(modifiers->flags & _DPRINTK_FLAGS_ENABLE) &&
| +				    !(newflags & _DPRINTK_FLAGS_ENABLE)) {
|  					static_branch_disable(&dp->key.dd_key_true);
| -			} else if (modifiers->flags & _DPRINTK_FLAGS_PRINT)
| +				}
| +			} else if (modifiers->flags & _DPRINTK_FLAGS_ENABLE) {
|  				static_branch_enable(&dp->key.dd_key_true);
| +			}
|  #endif