afaik integer increments are not atomic, so put inside the lock

src/alert-unified-alert.c

@@ -227,11 +227,11 @@ TmEcode AlertUnifiedAlert (ThreadVars *tv, Packet *p, void *data, PacketQueue *p
         /* force writing to disk so barnyard will not read half
          * written records and choke. */
         fflush(aun->file_ctx->fp);
-        SCMutexUnlock(&aun->file_ctx->fp_mutex);
 
         aun->file_ctx->size_current += sizeof(hdr);
+        aun->file_ctx->alerts++;
+        SCMutexUnlock(&aun->file_ctx->fp_mutex);
     }
-    aun->file_ctx->alerts += p->alerts.cnt;
 
     return TM_ECODE_OK;
 }

src/alert-unified-log.c

@@ -243,10 +243,11 @@ TmEcode AlertUnifiedLog (ThreadVars *tv, Packet *p, void *data, PacketQueue *pq)
         /* force writing to disk so barnyard will not read half
          * written records and choke. */
         fflush(aun->file_ctx->fp);
-        SCMutexUnlock(&aun->file_ctx->fp_mutex);
+
+        aun->file_ctx->alerts++;
         aun->file_ctx->size_current += buflen;
+        SCMutexUnlock(&aun->file_ctx->fp_mutex);
     }
-    aun->file_ctx->alerts += p->alerts.cnt;
 
     return TM_ECODE_OK;
 }

src/alert-unified2-alert.c

@@ -375,12 +375,12 @@ int Unified2IPv6TypeAlert (ThreadVars *t, Packet *p, void *data, PacketQueue *pq
         fflush(aun->file_ctx->fp);
 
         aun->file_ctx->size_current += len;
+        aun->file_ctx->alerts++;
 
         Unified2PacketTypeAlert(t, p, data);
         SCMutexUnlock(&aun->file_ctx->fp_mutex);
     }
 
-    aun->file_ctx->alerts += p->alerts.cnt;
 
     return 0;
 }