Hi Steve, Can the modified test program reproduce the problem in your test setup?
Thanks, --Martin On Tue, Jun 10, 2014 at 10:58:14PM -0700, Martin Lau wrote: > Hi Steve, > > Attached is the modified test program. Here is the sample output: > > localhost ~ # ./ftrace-test-epoll-kafai > <...>-1857 [000] ...1 720.174295: tracing_mark_write: some data > 1857: waitting for more data...... > 1858: written more data > <block here> > > Thanks, > --Martin > > On Tue, Jun 10, 2014 at 11:49:15AM -0400, Steven Rostedt wrote: > > On Mon, 9 Jun 2014 23:06:42 -0700 > > Martin Lau <[email protected]> wrote: > > > > > ring_buffer_poll_wait() should always put the poll_table to its wait_queue > > > even there is immediate data available. Otherwise, the following epoll > > > and > > > read sequence will eventually hang forever: > > > > > > 1. Put some data to make the trace_pipe ring_buffer read ready first > > > 2. epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, ee) > > > 3. epoll_wait() > > > 4. read(trace_pipe_fd) till EAGAIN > > > 5. Add some more data to the trace_pipe ring_buffer > > > 6. epoll_wait() -> this epoll_wait() will block forever > > > > > > ~ During the epoll_ctl(efd, EPOLL_CTL_ADD,...) call in step 2, > > > ring_buffer_poll_wait() returns immediately without adding poll_table, > > > which has poll_table->_qproc pointing to ep_poll_callback(), to its > > > wait_queue. > > > ~ During the epoll_wait() call in step 3 and step 6, > > > ring_buffer_poll_wait() cannot add ep_poll_callback() to its wait_queue > > > because the poll_table->_qproc is NULL and it is how epoll works. > > > ~ When there is new data available in step 6, ring_buffer does not know > > > it has to call ep_poll_callback() because it is not in its wait queue. > > > Hence, block forever. > > > > > > Other poll implementation seems to call poll_wait() unconditionally as > > > the very > > > first thing to do. For example, tcp_poll() in tcp.c. > > > > I'm trying to see the effect of this bug, but can't seem to reproduce > > it. Maybe I did something wrong. Attached is a test program I wrote > > trying to follow your instructions. I don't use epoll, so perhaps I > > didn't use it correctly. > > > > Can you modify it to show me the problem this is trying to fix. That > > is, without this patch it hangs, but with the patch it does not. > > > > Thanks! > > > > -- Steve > > > > > > > > Signed-off-by: Martin Lau <[email protected]> > > > --- > > > kernel/trace/ring_buffer.c | 4 ---- > > > 1 file changed, 4 deletions(-) > > > > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > > > index fd12cc5..a6e64e8 100644 > > > --- a/kernel/trace/ring_buffer.c > > > +++ b/kernel/trace/ring_buffer.c > > > @@ -613,10 +613,6 @@ int ring_buffer_poll_wait(struct ring_buffer > > > *buffer, int cpu, > > > struct ring_buffer_per_cpu *cpu_buffer; > > > struct rb_irq_work *work; > > > > > > - if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) || > > > - (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, > > > cpu))) > > > - return POLLIN | POLLRDNORM; > > > - > > > if (cpu == RING_BUFFER_ALL_CPUS) > > > work = &buffer->irq_work; > > > else { > > > > #include <stdio.h> > #include <stdlib.h> > #include <string.h> > #include <sys/types.h> > #include <sys/stat.h> > #include <sys/epoll.h> > #include <fcntl.h> > #include <unistd.h> > #include <assert.h> > > static const char * debugfs_list[] = { > "/debug/tracing", > "/sys/kernel/debug/tracing", > "/d/tracing", > NULL, > }; > > static const char *debugfs; > static int markfd; > static int trace_pipe_fd; > > static const char *find_debugfs(void) > { > struct stat st; > int i; > int r; > > for (i = 0; debugfs_list[i]; i++) { > r = stat(debugfs_list[i], &st); > if (r < 0) > continue; > if (S_ISDIR(st.st_mode)) > return debugfs_list[i]; > } > return NULL; > } > > static char * make_path(const char *file) > { > char *path; > int size; > > size = strlen(debugfs) + strlen(file) + 2; > path = malloc(size); > if (!path) { > perror("malloc"); > exit(-1); > } > sprintf(path, "%s/%s", debugfs, file); > return path; > } > > static void mark_write(const char *str) > { > write(markfd, str, strlen(str)); > } > > static void read_trace_pipe(void) > { > char buf[1024]; > int r; > > while ((r = read(trace_pipe_fd, buf, 1024)) > 0) > printf("%.*s", r, buf); > } > > int main (int argc, char **argv) > { > struct epoll_event ee; > char *marker; > char *pipe; > int efd; > int ret; > pid_t dwrt_pid; > > debugfs = find_debugfs(); > if (!debugfs) { > fprintf(stderr, "Could not find debugfs\n"); > exit(-1); > } > > marker = make_path("trace_marker"); > pipe = make_path("trace_pipe"); > > markfd = open(marker, O_WRONLY); > if (markfd < 0) { > perror("marker"); > exit(-1); > } > trace_pipe_fd = open(pipe, O_RDONLY|O_NONBLOCK); > if (trace_pipe_fd < 0) { > perror("trace_pipe"); > exit(-1); > } > > efd = epoll_create(1); > if (efd < 0) { > perror("epoll_create"); > exit(-1); > } > > mark_write("some data"); > ret = epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, &ee); > if (ret < 0) { > perror("epoll_ctl"); > exit(-1); > } > epoll_wait(efd, &ee, 1, -1); > read_trace_pipe(); > dwrt_pid = fork(); > assert(dwrt_pid != -1); > if (dwrt_pid == 0) { > sleep(10); > mark_write("more data"); > printf("%d: written more data\n", getpid()); > } else { > printf("%d: waitting for more data......\n", getpid()); > epoll_wait(efd, &ee, 1, -1); > printf("%d: got more data\n", getpid()); > read_trace_pipe(); > } > return 0; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
