From: Alan Huang <mmpgouride@gmail.com>
To: paulmck@kernel.org, akiyks@gmail.com
Cc: perfbook@vger.kernel.org, Alan Huang <mmpgouride@gmail.com>
Subject: [PATCH] CodeSamples/count: Remove unnecessary memory barriers
Date: Fri, 7 Apr 2023 13:58:13 -0400 [thread overview]
Message-ID: <20230407175813.1334028-1-mmpgouride@gmail.com> (raw)
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
next reply other threads:[~2023-04-07 17:58 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-07 17:58 Alan Huang [this message]
2023-04-07 18:04 ` [PATCH] CodeSamples/count: Remove unnecessary memory barriers Alan Huang
2023-04-09 0:11 ` Akira Yokosawa
2023-04-10 19:06 ` Paul E. McKenney
2023-04-11 16:34 ` Alan Huang
2023-04-12 11:32 ` Elad Lahav
2023-04-12 18:50 ` Paul E. McKenney
2023-04-08 4:25 ` Akira Yokosawa
2023-04-08 6:02 ` Alan Huang
2023-04-08 6:04 ` Alan Huang
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