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From: Alan Huang <mmpgouride@gmail.com>
To: paulmck@kernel.org, perfbook@vger.kernel.org
Cc: Alan Huang <mmpgouride@gmail.com>
Subject: [PATCH] CodeSamples/count: add necessary memory barrier
Date: Sun, 26 Mar 2023 12:28:28 -0400	[thread overview]
Message-ID: <20230326162828.165472-1-mmpgouride@gmail.com> (raw)

This patch add two necessary memory barriers, the first one added in
flush_local_count 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 memory barrier is added 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 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/CodeSamples/count/count_lim_sig.c b/CodeSamples/count/count_lim_sig.c
index 023d6215..7bfeb004 100644
--- a/CodeSamples/count/count_lim_sig.c
+++ b/CodeSamples/count/count_lim_sig.c
@@ -86,10 +86,12 @@ static void flush_local_count(void)			//\lnlbl{flush:b}
 			if (READ_ONCE(*theftp[t]) == THEFT_REQ)//\lnlbl{flush:check:REQ}
 				pthread_kill(tid, SIGUSR1);//\lnlbl{flush:signal2}
 		}					//\lnlbl{flush:loop3:e}
+		smp_mb();
 		globalcount += *counterp[t];		//\lnlbl{flush:thiev:b}
 		*counterp[t] = 0;
 		globalreserve -= *countermaxp[t];
 		*countermaxp[t] = 0;			//\lnlbl{flush:thiev:e}
+		smp_mb();
 		WRITE_ONCE(*theftp[t], THEFT_IDLE);	//\lnlbl{flush:IDLE}
 	}						//\lnlbl{flush:loop2:e}
 }							//\lnlbl{flush:e}
-- 
2.34.1

next             reply	other threads:[~2023-03-26 16:28 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-26 16:28 Alan Huang [this message]
2023-03-27  1:46 ` [PATCH] CodeSamples/count: add necessary memory barrier Akira Yokosawa

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