On Sat, 8 Apr 2023 01:57:53 -0400, Alan Huang wrote:
> 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@xxxxxxxxx>
v3 of this patch already has this, but to be clear,
Reviewed-by: Akira Yokosawa <akiyks@xxxxxxxxx>
Thanks, Akira
> ---
> 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..bb24631c 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 \co{counter} in
> +the fastpath happens before the change of \co{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 \co{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.
