From e1c32120ce6eddb72272b4717dd8384075b32c2f Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= Date: Wed, 8 May 2024 11:50:09 +0100 Subject: [PATCH] tests: fix hang in virshtest 'read-big-pipe' case MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The virshtest program testPipeFeeder method is doing this: mkfifo("test.fifo", 0600) ; int fd = open("test.fifo", O_RDWR); char buf[...]; memset(buf, 'a', sizeof(buf)); write(fd, buf, sizeof(buf)) == sizeof(buf)); close(fd); while the the 'virsh' child process then ends up doing: fd = open("test.fifo", O_RDONLY); read(fd, buf, sizeof(buf)) == sizeof(buf)); close(fd); The 'virsh' code hangs on open() on at least ppc64 and some other arches. It can be provoked to hang even on x86 by reducing the size of the buffer. It can be prevented from hanging on ppc64 by increasing the size of the buffer. What is happening is a result of differing page sizes, altering the overall pipe capacity size, since pipes on linux default to 16 pages in size and thus have architecture specific capacity when measured in bytes. * On x86, testPipeFeeder opens R+W, tries to write 140kb and write() blocks because the pipe is full. This gives time for virsh to start up, and it can open the pipe for O_RDONLY since testPipeFeeder still has it open for write. Everything works as intended. * On ppc64, testPipeFeeder opens R+W, tries to write 140kb and write() succeeds because the larger 64kb page size resulted in greater buffer capacity for the pipe. It thus quickly closes the pipe, removing the writer, and triggering discard of all the unread data. Now virsh starts up, tries to open the pipe for O_RDONLY and blocks waiting for a new writer to open it, which will never happen. Meson kills the test after 30 seconds. NB, every now & then, it will not block because virsh starts up quickly enough that testPipeFeeder has not yet closed the write end of the pipe, giving the illusion of correctness. The key flaw here is that it should not have been using O_RDWR in testPipeFeeder. Synchronization is required such that both virsh and testPipeFeeder have their respective ends of the pipe open before any data is sent. This is trivially arranged by using O_WRONLY in testPipeFeeder. Reviewed-by: Andrea Bolognani Signed-off-by: Daniel P. Berrangé --- tests/virshtest.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tests/virshtest.c b/tests/virshtest.c index a1ae481316..7a7797647c 100644 --- a/tests/virshtest.c +++ b/tests/virshtest.c @@ -145,7 +145,7 @@ testPipeFeeder(void *opaque) g_autofree char *doc = g_new0(char, emptyspace + xmlsize + 1); VIR_AUTOCLOSE fd = -1; - if ((fd = open(pipepath, O_RDWR)) < 0) { + if ((fd = open(pipepath, O_WRONLY)) < 0) { fprintf(stderr, "\nfailed to open pipe '%s': %s\n", pipepath, g_strerror(errno)); return; }