rseq: do 8 rseq attempts before using refcount fallback

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/liblttng-ust/lttng-ring-buffer-client.h b/liblttng-ust/lttng-ring-buffer-client.h
index 8a2155ef3e8501309948c49966fd37b9f41b8b52..1a15cfab0139e68af24baea6b3c0fed67e5e9c61 100644 (file)
--- a/liblttng-ust/lttng-ring-buffer-client.h
+++ b/liblttng-ust/lttng-ring-buffer-client.h
@@ -30,6 +30,7 @@
 
 #define LTTNG_COMPACT_EVENT_BITS       5
 #define LTTNG_COMPACT_TSC_BITS         27
+#define LTTNG_RSEQ_ATTEMPTS            8
 
 enum app_ctx_mode {
        APP_CTX_DISABLED,
@@ -718,7 +719,7 @@ int lttng_event_reserve(struct lttng_ust_lib_ring_buffer_ctx *ctx,
 {
        struct lttng_channel *lttng_chan = channel_get_private(ctx->chan);
        struct lttng_rseq_state rseq_state;
-       int ret, cpu;
+       int ret, cpu, attempt = 0;
        bool put_fallback_ref = false;
 
        if (lib_ring_buffer_begin(&client_config))
@@ -763,6 +764,10 @@ fallback:
        if (caa_unlikely(ret)) {
                if (ret == -EAGAIN) {
                        assert(!put_fallback_ref);
+                       if (++attempt < LTTNG_RSEQ_ATTEMPTS) {
+                               caa_cpu_relax();
+                               goto retry;
+                       }
                        put_fallback_ref = refcount_get_saturate(
                                &lttng_chan->chan->u.reserve_fallback_ref);
                        cpu = lib_ring_buffer_get_cpu(&client_config);