[PATCH v2] tcp: Add memory barrier to tcp_push()

[Salvatore Dipietro <dipiets@amazon.com>]

On CPUs with weak memory models, reads and updates performed by tcp_push to the
sk variables can get reordered leaving the socket throttled when it should not.
The tasklet running tcp_wfree() may also not observe the memory updates in time
and will skip flushing any packets throttled by tcp_push(), delaying the sending.
This can pathologically cause 40ms extra latency due to bad interactions with
delayed acks.

Modeling the memory access behavior of tcp_push() (P0) and tcp_wfree() (P1)
using the herd7 simulator, proves this behavior can occur. Below is the litmus
model which describes the functions:
```
C MP+tcp
{
  [flag] = 0;
  [sk] = 5;
  [corked] = 0;
}

P0(int *flag, int *sk, int *corked){
    int r0;
    int r1;
    int r2;

    r1 = READ_ONCE(*sk);
    if (r1 == 5) {

        r0 = READ_ONCE(*flag);
        if (r0 == 0) {
            WRITE_ONCE(*flag, 1);
        }

        // memory barrier added in this patch,
        // original code does not order the reads/writes
        smp_mb();

        r2 = READ_ONCE(*sk);
        if (r2 == 5 ) {
            WRITE_ONCE(*corked,1);
        }
    }
}

P1(int *flag, int *sk, int *corked){
    int r0;
    int r1;

    r1 = READ_ONCE(*sk);
    smp_store_release(sk, 0);

    r0 = smp_load_acquire(flag);
    if (r0 == 1) {
        smp_store_release(flag, 0);
    }
}
locations [0:r0; 0:r1; 0:r2; 1:r0; 1:r1; flag; sk; corked; ]
exists ( flag=1 /\ corked=1 )
```

Adding the memory barrier removes the positive witness from the memory model.
smp_mb__after_atomic() is used to not incur in unnecessary overhead on x86
since not affected.
Patch has been tested using an AWS c7g.2xlarge instance with Ubuntu 22.04 and
Apache Tomcat 9.0.83 running the basic servlet below:
```
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorldServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream(),"UTF-8");
        String s = "a".repeat(3096);
        osw.write(s,0,s.length());
        osw.flush();
    }
}
```
Load was applied using wrk2 (https://github.com/kinvolk/wrk2) from an AWS
c6i.8xlarge instance. Before the patch an additional 40ms latency from P99.99+
values is observed while, with the patch, the extra latency disappears.

# No patch and tcp_autocorking=1
./wrk -t32 -c128 -d40s --latency -R10000  http://172.31.60.173:8080/hello/hello
  ...
 50.000%    0.91ms
 75.000%    1.13ms
 90.000%    1.46ms
 99.000%    1.74ms
 99.900%    1.89ms
 99.990%   41.95ms  <<< 40+ ms extra latency
 99.999%   48.32ms
100.000%   48.96ms

# With patch and tcp_autocorking=1
./wrk -t32 -c128 -d40s --latency -R10000  http://172.31.60.173:8080/hello/hello
  ...
 50.000%    0.90ms
 75.000%    1.13ms
 90.000%    1.45ms
 99.000%    1.72ms
 99.900%    1.83ms
 99.990%    2.11ms  <<< no 40+ ms extra latency
 99.999%    2.53ms
100.000%    2.62ms

Patch has been also tested on x86 (m7i.2xlarge instance) which it is not
affected by this issue and the patch doesn't introduce any additional
delay.

Fixes: f54b311142a9 ("tcp: auto corking")
Signed-off-by: Salvatore Dipietro <dipiets@amazon.com>
---
 net/ipv4/tcp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ff6838ca2e58..ab9e3922393c 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -726,6 +726,7 @@ void tcp_push(struct sock *sk, int flags, int mss_now,
 		/* It is possible TX completion already happened
 		 * before we set TSQ_THROTTLED.
 		 */
+		smp_mb__after_atomic();
 		if (refcount_read(&sk->sk_wmem_alloc) > skb->truesize)
 			return;
 	}
-- 
2.42.0