Re: [PATCH] CodeSamples/count: Remove unnecessary memory barriers

[Alan Huang <mmpgouride@gmail.com>]

Hi Paul and Akira,

This is the patch v3, I forgot to add a version tag…

And I think it may be better to add a question in a subsequent path.

Thanks,
Alan



> 2023年4月8日 上午1:58，Alan Huang <mmpgouride@gmail.com> 写道：
> 
> In count_lim_sig.c, there is only one ordering required, that is
> writing to counter happens before setting theft to THEFT_READY in
> add_count/sub_count's fast path. Therefore, partial memory barrier
> will suffice.
> 
> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
> ---
> CodeSamples/count/count_lim_sig.c | 10 +++-------
> count/count.tex                   | 12 ++++++------
> 2 files changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/CodeSamples/count/count_lim_sig.c b/CodeSamples/count/count_lim_sig.c
> index 59da8077..c2f61197 100644
> --- a/CodeSamples/count/count_lim_sig.c
> +++ b/CodeSamples/count/count_lim_sig.c
> @@ -56,12 +56,10 @@ static void flush_local_count_sig(int unused)	//\lnlbl{flush_sig:b}
> {
> 	if (READ_ONCE(theft) != THEFT_REQ)	//\lnlbl{flush_sig:check:REQ}
> 		return;				//\lnlbl{flush_sig:return:n}
> -	smp_mb();				//\lnlbl{flush_sig:mb:1}
> 	WRITE_ONCE(theft, THEFT_ACK);		//\lnlbl{flush_sig:set:ACK}
> 	if (!counting) {			//\lnlbl{flush_sig:check:fast}
> -		WRITE_ONCE(theft, THEFT_READY);	//\lnlbl{flush_sig:set:READY}
> +		smp_store_release(&theft, THEFT_READY);	//\lnlbl{flush_sig:set:READY}
> 	}
> -	smp_mb();
> }						//\lnlbl{flush_sig:e}
> 
> static void flush_local_count(void)			//\lnlbl{flush:b}
> @@ -125,8 +123,7 @@ int add_count(unsigned long delta)			//\lnlbl{b}
> 	WRITE_ONCE(counting, 0);			//\lnlbl{clearcnt}
> 	barrier();					//\lnlbl{barrier:3}
> 	if (READ_ONCE(theft) == THEFT_ACK) {		//\lnlbl{check:ACK}
> -		smp_mb();				//\lnlbl{mb}
> -		WRITE_ONCE(theft, THEFT_READY);		//\lnlbl{READY}
> +		smp_store_release(&theft, THEFT_READY);		//\lnlbl{READY}
> 	}
> 	if (fastpath)
> 		return 1;				//\lnlbl{return:fs}
> @@ -164,8 +161,7 @@ int sub_count(unsigned long delta)
> 	WRITE_ONCE(counting, 0);
> 	barrier();
> 	if (READ_ONCE(theft) == THEFT_ACK) {
> -		smp_mb();
> -		WRITE_ONCE(theft, THEFT_READY);
> +		smp_store_release(&theft, THEFT_READY);
> 	}
> 	if (fastpath)
> 		return 1;
> diff --git a/count/count.tex b/count/count.tex
> index 8ab67e2e..4c139d63 100644
> --- a/count/count.tex
> +++ b/count/count.tex
> @@ -2425,12 +2425,11 @@ handler used in the theft process.
> \Clnref{check:REQ,return:n} check to see if
> the \co{theft} state is REQ, and, if not
> returns without change.
> -\Clnref{mb:1} executes a \IX{memory barrier} to ensure that the sampling
> -of the theft variable happens before any change to that variable.
> \Clnref{set:ACK} sets the \co{theft} state to ACK, and, if
> \clnref{check:fast} sees that
> this thread's fastpaths are not running, \clnref{set:READY} sets the \co{theft}
> -state to READY\@.
> +state to READY, with the release store ensuring any change to counter in
> +the fastpath happens before the change of theft to READY\@.
> \end{fcvref}
> 
> \begin{listing}
> @@ -2595,9 +2594,10 @@ handlers to undertake theft.
> \Clnref{barrier:3} again disables compiler reordering, and then
> \clnref{check:ACK}
> checks to see if the signal handler deferred the \co{theft}
> -state-change to READY, and, if so, \clnref{mb} executes a memory
> -barrier to ensure that any CPU that sees \clnref{READY} setting state to
> -READY also sees the effects of \clnref{add:f}.
> +state-change to READY, and, if so, \clnref{READY} changes theft to
> +READY with the release store ensuring that
> +any CPU that sees the READY state also sees the effects
> +of \clnref{add:f}.
> If the fastpath addition at \clnref{add:f} was executed, then
> \clnref{return:fs} returns
> success.
> -- 
> 2.34.1
>