On Tue, 4 Apr 2023 12:47:53 -0400, Alan Huang wrote:
> The original memory barriers in count_stat_eventual.c ensure
> writing to global_count happens before writing to stopflag and
> reading from stopflag happens before later reading from global_count.
>
> Thus, smp_load_acquire and smp_store_release will suffice.
>
> 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.
Hi Alan,
My comment on count_lim_sig.c here. See below
>
> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
> ---
> CodeSamples/count/count_lim_sig.c | 10 +++-------
> CodeSamples/count/count_stat_eventual.c | 6 ++----
> count/count.tex | 9 ++++-----
> 3 files changed, 9 insertions(+), 16 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}
You see, this a signal handler.
It can interrupt anywhere in normal threads.
That is why you see a lot of barrier() calls in updater functions.
This code has no assumption on memory barriers around signal handlers.
So, I'd rather keep smp_mb()s in this function.
Do you have a good reasoning why the pair of smp_mb()s can be safely
replaced with the single smp_store_release() ?
>
> 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;
These changes in updater functions look good to me.
I'm stopping here. Let's reach a consensus on code changes first.
Thanks, Akira
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