Libguestfs - Mar 2020 - Re: [nbdkit PATCH 2/2] nbd: Reorder cleanup to avoid getting stuck in poll()

I still need more soak time on testing, to see whether I can:
a) reproduce the hang with patch 2 not applied
b) feel confident that patch 2 is sufficient to fix the race, or else
determine that I also need to augment the loop condition in the reader
thread to additionally break out of the loop when the pipe-to-self
sees EOF even when nbd_aio_is_dead() has not yet been satisfied

I'm also fairly confident that I'll need to patch libnbd to call
shutdown(SHUT_WR) on plaintext and gnutls_bye(GNUTLS_SHUT_WR) on TLS
connections after sending NBD_CMD_DISC, as that is something we lowt
when the nbd plugin switched to libnbd, even after documenting in
commit 430f8141 why it is important for preventing shutdown deadlocks.

Eric Blake (2):
  nbd: Don't reference stale errno in reader loop
  nbd: Reorder cleanup to avoid getting stuck in poll()

 plugins/nbd/nbd.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

-- 
2.26.0.rc2

When switching to libnbd in commit ab7760fcfd, I mistakenly assumed
that after a POLLIN event fires on the pipe-to-self, then read() will
return either 1 or -1.  But this is not true; read() can also return 0
(if the pipe hits EOF), in which case POSIX says errno has an
unspecified value, and we should not be deciding whether to log an
error based on a random value.  I did not manage to fix the bug even
when later refactoring the pipe-to-self to be non-blocking, although
it appears to be latent as long as the pipe is non-blocking and the
write end is not closed.

Fixes: e0d32468
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 plugins/nbd/nbd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
index d020beec..e5b8f338 100644
--- a/plugins/nbd/nbd.c
+++ b/plugins/nbd/nbd.c
@@ -332,9 +332,11 @@ nbdplug_reader (void *handle)

     /* Check if we were kicked because a command was started */
     if (fds[1].revents & POLLIN) {
-      while (read (h->fds[0], &c, 1) == 1)
+      int r;
+
+      while ((r = read (h->fds[0], &c, 1)) == 1)
         /* Drain any backlog */;
-      if (errno != EAGAIN) {
+      if (r == -1 && errno != EAGAIN) {
         nbdkit_error ("failed to read pipe: %m");
         break;
       }
-- 
2.26.0.rc2

We have been seeing sporadic hangs on test-nbd-tls-psk.sh, where even
though the client to the 'nbdkit nbd' process has cleanly exited,
things are stalled in .close where nbd is trying to pthread_join() the
reader thread, while the reader thread is itself blocked on a poll()
that will never make additional progress.  Tracing the race is
difficult: nbd_shutdown() sends NBD_CMD_DISC to the server, and the
NBD protocol does not require the server to send a response but does
not forbid the server from using gnutls_bye to at least gracefully end
the TLS session.  So where a plaintext server just closes the socket
(and the resulting EOF exits our poll()), it appears that with TLS, we
can sometimes hit the situation where the server is waiting for a
response to gnutls_bye() (note that qemu does not use this function,
but nbdkit does), while at the same time our client is waiting to see
EOF on the socket, resulting in deadlock.  In the past (see commit
430f8141), we added shutdown(SHUT_WR) for plaintext clients to prod
the server into closing the socket faster, but libnbd does not yet
have that counterpart action.

But I can at least reuse the same mechanism we have for waking up the
poll() loop when first sending a command to the server.  That is,
since we already have a pipe-to-self in addition to reading from the
server, it's trivial to argue that closing the pipe-to-self will
guarantee that the reader thread sees something interesting to break
out of its poll() loop, regardless of whether it also sees something
interesting from the server after having sent NBD_CMD_DISC, and
regardless of whether I need to add in more gnutls_bye() calls to
either nbdkit or libnbd.

Fixes: ab7760fc
Signed-off-by: Eric Blake <eblake@redhat.com>
---

May be incomplete: I might also need to break out of the reader loop
when read() returns 0.

 plugins/nbd/nbd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
index e5b8f338..23a7da06 100644
--- a/plugins/nbd/nbd.c
+++ b/plugins/nbd/nbd.c
@@ -537,10 +537,10 @@ nbdplug_close_handle (struct handle *h)
 {
   if (nbd_shutdown (h->nbd, 0) == -1)
     nbdkit_debug ("failed to clean up handle: %s", nbd_get_error ());
+  close (h->fds[1]);
   if ((errno = pthread_join (h->reader, NULL)))
     nbdkit_debug ("failed to join reader thread: %m");
   close (h->fds[0]);
-  close (h->fds[1]);
   nbd_close (h->nbd);
   free (h);
 }
-- 
2.26.0.rc2

On Fri, Mar 27, 2020 at 05:33:27PM -0500, Eric Blake
wrote:
> When switching to libnbd in commit ab7760fcfd, I mistakenly assumed
> that after a POLLIN event fires on the pipe-to-self, then read() will
> return either 1 or -1.  But this is not true; read() can also return 0
> (if the pipe hits EOF), in which case POSIX says errno has an
> unspecified value, and we should not be deciding whether to log an
> error based on a random value.  I did not manage to fix the bug even
> when later refactoring the pipe-to-self to be non-blocking, although
> it appears to be latent as long as the pipe is non-blocking and the
> write end is not closed.
> 
> Fixes: e0d32468
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  plugins/nbd/nbd.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
> index d020beec..e5b8f338 100644
> --- a/plugins/nbd/nbd.c
> +++ b/plugins/nbd/nbd.c
> @@ -332,9 +332,11 @@ nbdplug_reader (void *handle)
> 
>      /* Check if we were kicked because a command was started */
>      if (fds[1].revents & POLLIN) {
> -      while (read (h->fds[0], &c, 1) == 1)
> +      int r;
> +
> +      while ((r = read (h->fds[0], &c, 1)) == 1)
>          /* Drain any backlog */;
> -      if (errno != EAGAIN) {
> +      if (r == -1 && errno != EAGAIN) {
>          nbdkit_error ("failed to read pipe: %m");
>          break;
>        }
This one seems pretty obvious, so ACK.

Although personally I would have made one other change: The semicolon
after the comment was invisible to me until I had studied the code
above and the original code for a while.  I actually wrote a quite
lengthy email here, which was completely wrong once I noticed the
semicolon.  Can we make that more visible somehow?

Rich.

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On Fri, Mar 27, 2020 at 05:33:28PM -0500, Eric Blake
wrote:
> We have been seeing sporadic hangs on test-nbd-tls-psk.sh, where even
> though the client to the 'nbdkit nbd' process has cleanly exited,
> things are stalled in .close where nbd is trying to pthread_join() the
> reader thread, while the reader thread is itself blocked on a poll()
> that will never make additional progress.  Tracing the race is
> difficult: nbd_shutdown() sends NBD_CMD_DISC to the server, and the
> NBD protocol does not require the server to send a response but does
> not forbid the server from using gnutls_bye to at least gracefully end
> the TLS session.  So where a plaintext server just closes the socket
> (and the resulting EOF exits our poll()), it appears that with TLS, we
> can sometimes hit the situation where the server is waiting for a
> response to gnutls_bye() (note that qemu does not use this function,
> but nbdkit does), while at the same time our client is waiting to see
> EOF on the socket, resulting in deadlock.  In the past (see commit
> 430f8141), we added shutdown(SHUT_WR) for plaintext clients to prod
> the server into closing the socket faster, but libnbd does not yet
> have that counterpart action.
> 
> But I can at least reuse the same mechanism we have for waking up the
> poll() loop when first sending a command to the server.  That is,
> since we already have a pipe-to-self in addition to reading from the
> server, it's trivial to argue that closing the pipe-to-self will
> guarantee that the reader thread sees something interesting to break
> out of its poll() loop, regardless of whether it also sees something
> interesting from the server after having sent NBD_CMD_DISC, and
> regardless of whether I need to add in more gnutls_bye() calls to
> either nbdkit or libnbd.
> 
> Fixes: ab7760fc
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 
> May be incomplete: I might also need to break out of the reader loop
> when read() returns 0.
> 
>  plugins/nbd/nbd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
> index e5b8f338..23a7da06 100644
> --- a/plugins/nbd/nbd.c
> +++ b/plugins/nbd/nbd.c
> @@ -537,10 +537,10 @@ nbdplug_close_handle (struct handle *h)
>  {
>    if (nbd_shutdown (h->nbd, 0) == -1)
>      nbdkit_debug ("failed to clean up handle: %s", nbd_get_error
());
> +  close (h->fds[1]);
>    if ((errno = pthread_join (h->reader, NULL)))
>      nbdkit_debug ("failed to join reader thread: %m");
>    close (h->fds[0]);
> -  close (h->fds[1]);
>    nbd_close (h->nbd);
>    free (h);
>  }
Probably deserves a comment in the code to mention to future Eric (and
Richard) not to move that close call.  But yes this is also fine, so ACK.

Rich.

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