[bpf-next] samples/bpf: xdpsock: Fix race when running for fix duration of time

[bpf-next] samples/bpf: xdpsock: Fix race when running for fix duration of time

[Niklas Söderlund]

When running xdpsock for a fix duration of time before terminating
using --duration=<n>, there is a race condition that may cause xdpsock
to terminate immediately.

When running for a fixed duration of time the check to determine when to
terminate execution is in is_benchmark_done() and is being executed in
the context of the poller thread,

    if (opt_duration > 0) {
            unsigned long dt = (get_nsecs() - start_time);

            if (dt >= opt_duration)
                    benchmark_done = true;
    }

However start_time is only set after the poller thread have been
created. This leaves a small window when the poller thread is starting
and calls is_benchmark_done() for the first time that start_time is not
yet set. In that case start_time have its initial value of 0 and the
duration check fails as it do not correlate correctly for the
applications start time and immediately sets benchmark_done which in
turn terminates the xdpsock application.

Fix this by setting start_time before creating the poller thread.

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
---
 samples/bpf/xdpsock_user.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/samples/bpf/xdpsock_user.c b/samples/bpf/xdpsock_user.c
index 19288a2bbc756d3f..6f3fe30ad283cf0a 100644
--- a/samples/bpf/xdpsock_user.c
+++ b/samples/bpf/xdpsock_user.c
@@ -1984,15 +1984,15 @@ int main(int argc, char **argv)
 
 	setlocale(LC_ALL, "");
 
+	prev_time = get_nsecs();
+	start_time = prev_time;
+
 	if (!opt_quiet) {
 		ret = pthread_create(&pt, NULL, poller, NULL);
 		if (ret)
 			exit_with_error(ret);
 	}
 
-	prev_time = get_nsecs();
-	start_time = prev_time;
-
 	/* Configure sched priority for better wake-up accuracy */
 	memset(&schparam, 0, sizeof(schparam));
 	schparam.sched_priority = opt_schprio;
-- 
2.35.1

Re: [bpf-next] samples/bpf: xdpsock: Fix race when running for fix duration of time

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Tue, 15 Mar 2022 11:29:48 +0100 you wrote:
> When running xdpsock for a fix duration of time before terminating
> using --duration=<n>, there is a race condition that may cause xdpsock
> to terminate immediately.
> 
> When running for a fixed duration of time the check to determine when to
> terminate execution is in is_benchmark_done() and is being executed in
> the context of the poller thread,
> 
> [...]

Here is the summary with links:
  - [bpf-next] samples/bpf: xdpsock: Fix race when running for fix duration of time
    https://git.kernel.org/bpf/bpf-next/c/8fa42d78f635

You are awesome, thank you!
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