Messages in this thread Patch in this message | | | Date | Tue, 10 Jun 2014 22:58:14 -0700 | From | Martin Lau <> | Subject | Re: [PATCH] ring-buffer: Fix polling on trace_pipe |
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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 <kafai@fb.com> 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 <kafai@fb.com> > > --- > > 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; }
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