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  • Anton Anton anjo
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  1. Dec 19, 2017
    • Stefan Hajnoczi's avatar
      iothread: fix iothread_stop() race condition · 2362a28e
      Stefan Hajnoczi authored 
      
There is a small chance that iothread_stop() hangs as follows:

  Thread 3 (Thread 0x7f63eba5f700 (LWP 16105)):
  #0  0x00007f64012c09b6 in ppoll () at /lib64/libc.so.6
  #1  0x000055959992eac9 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:77
  #2  0x000055959992eac9 in qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=<optimized out>) at util/qemu-timer.c:322
  #3  0x0000559599930711 in aio_poll (ctx=0x55959bdb83c0, blocking=blocking@entry=true) at util/aio-posix.c:629
  #4  0x00005595996806fe in iothread_run (opaque=0x55959bd78400) at iothread.c:59
  #5  0x00007f640159f609 in start_thread () at /lib64/libpthread.so.0
  #6  0x00007f64012cce6f in clone () at /lib64/libc.so.6

  Thread 1 (Thread 0x7f640b45b280 (LWP 16103)):
  #0  0x00007f64015a0b6d in pthread_join () at /lib64/libpthread.so.0
  #1  0x00005595999332ef in qemu_thread_join (thread=<optimized out>) at util/qemu-thread-posix.c:547
  #2  0x00005595996808ae in iothread_stop (iothread=<optimized out>) at iothread.c:91
  #3  0x000055959968094d in iothread_stop_iter (object=<optimized out>, opaque=<optimized out>) at iothread.c:102
  #4  0x0000559599857d97 in do_object_child_foreach (obj=obj@entry=0x55959bdb8100, fn=fn@entry=0x559599680930 <iothread_stop_iter>, opaque=opaque@entry=0x0, recurse=recurse@entry=false) at qom/object.c:852
  #5  0x0000559599859477 in object_child_foreach (obj=obj@entry=0x55959bdb8100, fn=fn@entry=0x559599680930 <iothread_stop_iter>, opaque=opaque@entry=0x0) at qom/object.c:867
  #6  0x0000559599680a6e in iothread_stop_all () at iothread.c:341
  #7  0x000055959955b1d5 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4913

The relevant code from iothread_run() is:

  while (!atomic_read(&iothread->stopping)) {
      aio_poll(iothread->ctx, true);

and iothread_stop():

  iothread->stopping = true;
  aio_notify(iothread->ctx);
  ...
  qemu_thread_join(&iothread->thread);

The following scenario can occur:

1. IOThread:
  while (!atomic_read(&iothread->stopping)) -> stopping=false

2. Main loop:
  iothread->stopping = true;
  aio_notify(iothread->ctx);

3. IOThread:
  aio_poll(iothread->ctx, true); -> hang

The bug is explained by the AioContext->notify_me doc comments:

  "If this field is 0, everything (file descriptors, bottom halves,
  timers) will be re-evaluated before the next blocking poll(), thus the
  event_notifier_set call can be skipped."

The problem is that "everything" does not include checking
iothread->stopping.  This means iothread_run() will block in aio_poll()
if aio_notify() was called just before aio_poll().

This patch fixes the hang by replacing aio_notify() with
aio_bh_schedule_oneshot().  This makes aio_poll() or g_main_loop_run()
to return.

Implementing this properly required a new bool running flag.  The new
flag prevents races that are tricky if we try to use iothread->stopping.
Now iothread->stopping is purely for iothread_stop() and
iothread->running is purely for the iothread_run() thread.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Message-id: 20171207201320.19284-6-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      2362a28e
    • Stefan Hajnoczi's avatar
      iothread: add iothread_by_id() API · fbcc6923
      Stefan Hajnoczi authored 
      
Encapsulate IOThread QOM object lookup so that callers don't need to
know how and where IOThread objects live.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarKevin Wolf <kwolf@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Message-id: 20171206144550.22295-8-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      fbcc6923
  3. Oct 03, 2017
  5. Sep 29, 2017
  7. Sep 08, 2017
  9. Feb 21, 2017
  11. Feb 03, 2017
    • Stefan Hajnoczi's avatar
      iothread: enable AioContext polling by default · cdd7abfd
      Stefan Hajnoczi authored 
      
IOThread AioContexts are likely to consist only of event sources like
virtqueue ioeventfds and LinuxAIO completion eventfds that are pollable
from userspace (without system calls).

We recently merged the AioContext polling feature but didn't enable it
by default yet.  I have gone back over the performance data on the
mailing list and picked a default polling value that gave good results.

Let's enable AioContext polling by default so users don't have another
switch they need to set manually.  If performance regressions are found
we can still disable this for the QEMU 2.9 release.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Karl Rister <krister@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170126170119.27876-1-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      cdd7abfd
  13. Jan 03, 2017
  15. Oct 28, 2016
  17. Sep 28, 2016
  19. Sep 13, 2016
    • Fam Zheng's avatar
      iothread: Stop threads before main() quits · dce8921b
      Fam Zheng authored 
      
Right after main_loop ends, we release various things but keep iothread
alive. The latter is not prepared to the sudden change of resources.

Specifically, after bdrv_close_all(), virtio-scsi dataplane get a
surprise at the empty BlockBackend:

(gdb) bt
    at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:543
    at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:577

It is because the d->conf.blk->root is set to NULL, then
blk_get_aio_context() returns qemu_aio_context, whereas s->ctx is still
pointing to the iothread:

    hw/scsi/virtio-scsi.c:543:

    if (s->dataplane_started) {
        assert(blk_get_aio_context(d->conf.blk) == s->ctx);
    }

To fix this, let's stop iothreads before doing bdrv_close_all().

Cc: qemu-stable@nongnu.org
Signed-off-by: default avatarFam Zheng <famz@redhat.com>
Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-id: 1473326931-9699-1-git-send-email-famz@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      dce8921b
  21. Feb 04, 2016
    • Peter Maydell's avatar
      all: Clean up includes · d38ea87a
      Peter Maydell authored 
      
Clean up includes so that osdep.h is included first and headers
which it implies are not included manually.

This commit was created with scripts/clean-includes.

Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
Message-id: 1454089805-5470-16-git-send-email-peter.maydell@linaro.org
      d38ea87a
  23. Dec 03, 2015
  25. Jul 24, 2015
  27. Jun 19, 2015
  29. Jun 12, 2015
  31. May 08, 2015
  33. Apr 28, 2015
  35. Sep 22, 2014
    • Chrysostomos Nanakos's avatar
      async: aio_context_new(): Handle event_notifier_init failure · 2f78e491
      Chrysostomos Nanakos authored 
      
On a system with a low limit of open files the initialization
of the event notifier could fail and QEMU exits without printing any
error information to the user.

The problem can be easily reproduced by enforcing a low limit of open
files and start QEMU with enough I/O threads to hit this limit.

The same problem raises, without the creation of I/O threads, while
QEMU initializes the main event loop by enforcing an even lower limit of
open files.

This commit adds an error message on failure:

 # qemu [...] -object iothread,id=iothread0 -object iothread,id=iothread1
 qemu: Failed to initialize event notifier: Too many open files in system

Signed-off-by: default avatarChrysostomos Nanakos <cnanakos@grnet.gr>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      2f78e491
  37. Jul 14, 2014
    • Paolo Bonzini's avatar
      AioContext: do not rely on aio_poll(ctx, true) result to end a loop · acfb23ad
      Paolo Bonzini authored 
      
Currently, whenever aio_poll(ctx, true) has completed all pending
work it returns true *and* the next call to aio_poll(ctx, true)
will not block.

This invariant has its roots in qemu_aio_flush()'s implementation
as "while (qemu_aio_wait()) {}".  However, qemu_aio_flush() does
not exist anymore and bdrv_drain_all() is implemented differently;
and this invariant is complicated to maintain and subtly different
from the return value of GMainLoop's g_main_context_iteration.

All calls to aio_poll(ctx, true) except one are guarded by a
while() loop checking for a request to be incomplete, or a
BlockDriverState to be idle.  The one remaining call (in
iothread.c) uses this to delay the aio_context_release/acquire
pair until the AioContext is quiescent, however:

- we can do the same just by using non-blocking aio_poll,
  similar to how vl.c invokes main_loop_wait

- it is buggy, because it does not ensure that the AioContext
  is released between an aio_notify and the next time the
  iothread goes to sleep.  This leads to hangs when stopping
  the dataplane thread.

In the end, these semantics are a bad match for the current
users of AioContext.  So modify that one exception in iothread.c,
which also fixes the hangs, as well as the testcase so that
it use the same idiom as the actual QEMU code.

Reported-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Tested-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
      acfb23ad
  39. Apr 04, 2014
  41. Mar 13, 2014
    • Stefan Hajnoczi's avatar
      qmp: add query-iothreads command · dc3dd0d2
      Stefan Hajnoczi authored 
      
The "query-iothreads" command returns a list of information about
iothreads.  See the patch for API documentation.

Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      dc3dd0d2
    • Stefan Hajnoczi's avatar
      iothread: stash thread ID away · 88eb7c29
      Stefan Hajnoczi authored 
      
Keep the thread ID around so we can report it via QMP.

There's only one problem: qemu_get_thread_id() (gettid() wrapper on
Linux) must be called from the thread itself.  There is no way to get
the thread ID outside the thread.

This patch uses a condvar to wait for iothread_run() to populate the
thread_id inside the thread.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      88eb7c29
    • Stefan Hajnoczi's avatar
      iothread: add I/O thread object · be8d8537
      Stefan Hajnoczi authored 
      
This is a stand-in for Michael Roth's QContext.  I expect this to be
replaced once QContext is completed.

The IOThread object is an AioContext event loop thread.  This patch adds
the concept of multiple event loop threads, allowing users to define
them.

When SMP guests run on SMP hosts it makes sense to instantiate multiple
IOThreads.  This spreads event loop processing across multiple cores.
Note that additional patches are required to actually bind a device to
an IOThread.

[Andreas Färber <afaerber@suse.de> pointed out that the embedded parent
object instance should be called "parent_obj" and have a newline
afterwards.  This patch has been changed to reflect this.
-- Stefan]

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      be8d8537