From: Alan Huang <mmpgouride@gmail.com>
To: akiyks@gmail.com
Cc: paulmck@kernel.org, perfbook@vger.kernel.org
Subject: Re: [PATCH v2] CodeSamples/count: add necessary partial memory barriers
Date: Tue, 28 Mar 2023 10:02:58 +0800 [thread overview]
Message-ID: <322EF303-5C92-4271-A449-4128D8A6EEC2@gmail.com> (raw)
In-Reply-To: <20230327164235.399487-1-mmpgouride@gmail.com>
Found another issue, since we store release the theft variable, we should load acquire theft in add_count/sub_count.
I will send patch v3.
Thanks,
Alan
> 在 2023年3月28日，00:42，Alan Huang <mmpgouride@gmail.com> 写道：
>
> This patch add two necessary partial memory barriers, the first one change
> READ_ONCE to smp_load_acquire to makes sure the reading from theftp[t]
> happens before the reading from counterp[t]. The litmus testing below
> represents the pattern, and the result is "Sometimes":
>
> C counter_sig
>
> {}
>
> P0(int *theft, int *counter)
> {
> int r0;
> int r1;
>
> r0 = READ_ONCE(*theft);
> r1 = READ_ONCE(*counter);
> }
>
> P1(int *theft, int *counter)
> {
> WRITE_ONCE(*counter, 1);
> smp_mb();
> WRITE_ONCE(*theft, 1);
> }
>
> exists (0:r0=1 /\ 0:r1=0)
>
> Second one change WRITE_ONCE to smp_store_release to make sure that setting
> counterp[t] happens before the setting theftp[p] to THEFT_IDLE. Here is the
> litmus testing, The result is "Sometimes":
>
> C counter_sig_2
>
> {
> int theft = 1;
> int counter = 1;
> }
>
> P0(int *theft, int *counter)
> {
> WRITE_ONCE(*counter, 0);
> WRITE_ONCE(*theft, 0);
> }
>
> P1(int *theft, int *counter)
> {
> if (READ_ONCE(*theft) == 0)
> {
> smp_mb();
> WRITE_ONCE(*counter, READ_ONCE(*counter)+1);
> }
> }
>
> P2(int *counter)
> {
> int r0;
>
> r0 = READ_ONCE(*counter);
> }
>
> exists (2:r0=2)
>
> Note that I added one smp_mb() in P1's "if" statement,
> because in add_count/sub_count's fast path, there is a
> control dependency.
>
> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
> ---
> CodeSamples/count/count_lim_sig.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/CodeSamples/count/count_lim_sig.c b/CodeSamples/count/count_lim_sig.c
> index 023d6215..09582429 100644
> --- a/CodeSamples/count/count_lim_sig.c
> +++ b/CodeSamples/count/count_lim_sig.c
> @@ -81,7 +81,7 @@ static void flush_local_count(void) //\lnlbl{flush:b}
> for_each_tid(t, tid) { //\lnlbl{flush:loop2:b}
> if (theftp[t] == NULL) //\lnlbl{flush:skip:nonexist}
> continue; //\lnlbl{flush:next2}
> - while (READ_ONCE(*theftp[t]) != THEFT_READY) {//\lnlbl{flush:loop3:b}
> + while (smp_load_acquire(theftp[t]) != THEFT_READY) {//\lnlbl{flush:loop3:b}
> poll(NULL, 0, 1); //\lnlbl{flush:block}
> if (READ_ONCE(*theftp[t]) == THEFT_REQ)//\lnlbl{flush:check:REQ}
> pthread_kill(tid, SIGUSR1);//\lnlbl{flush:signal2}
> @@ -90,7 +90,7 @@ static void flush_local_count(void) //\lnlbl{flush:b}
> *counterp[t] = 0;
> globalreserve -= *countermaxp[t];
> *countermaxp[t] = 0; //\lnlbl{flush:thiev:e}
> - WRITE_ONCE(*theftp[t], THEFT_IDLE); //\lnlbl{flush:IDLE}
> + smp_store_release(theftp[t], THEFT_IDLE); //\lnlbl{flush:IDLE}
> } //\lnlbl{flush:loop2:e}
> } //\lnlbl{flush:e}
>
> --
> 2.34.1
>
prev parent reply other threads:[~2023-03-28 2:02 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-27 16:42 [PATCH v2] CodeSamples/count: add necessary partial memory barriers Alan Huang
2023-03-28 2:02 ` Alan Huang [this message]
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