From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; t=1680890303; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=QGB1JH/2PiBEjcozcDIyM2i7LC3SDflrBbIcOZG4tfQ=; b=Wn+B3pL6NkGY/8xOZ46LSkqdIMFM0/O3UCcSxjc/4NOOQ54cffskv4PsOKEnBtiBYc IfyExmZDA8MULO2Wq6UKnIM78Bk6Ey3BIbFNDYS06nPU/jrb8t99RGOD3FX5ySVKxuzE rle3FthEROCh+dnYveKwVpjFMXyPhZlWpRMXnbTCARxKffgyO9gZHuGwGaTDDGaUPjbK owdSbV5F8jMt18zGaZ4+qvNECw/HA0wPdcYB5BF7yI8VArRl2Y9hvpXYi8EPfhU/l1PE qJj7Os40/qMf+RtbPQG4M1n4RYzz1TB9YWKXhPokyFizWAI4d0ol5M2gm6fd1gcCYaHY UGEw== From: Alan Huang Subject: [PATCH] CodeSamples/count: Remove unnecessary memory barriers Date: Fri, 7 Apr 2023 13:58:13 -0400 Message-Id: <20230407175813.1334028-1-mmpgouride@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit To: paulmck@kernel.org, akiyks@gmail.com Cc: perfbook@vger.kernel.org, Alan Huang List-ID: 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 --- 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