On Mon, 22 Aug 2016 17:02:41 +0200,
Ismael Luceno wrote:
>
> On 22/Ago/2016 11:26, Takashi Iwai wrote:
> > On Sun, 14 Aug 2016 02:28:52 +0200,
> > Ismael Luceno wrote:
> > >
> > > Easily seen when two threads try at the same time, one of them will fail.
> > >
> > > The bug was identified by using apulse with Skype.
> > >
> > > Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> > > Fixes: https://github.com/i-rinat/apulse/issues/38
> > > Signed-off-by: Ismael Luceno <ismael@xxxxxxxxxxx>
> > > ---
> > > src/pcm/pcm_direct.c | 2 ++
> > > 1 file changed, 2 insertions(+)
> > >
> > > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> > > index c3925cc20fd3..b5215ba35406 100644
> > > --- a/src/pcm/pcm_direct.c
> > > +++ b/src/pcm/pcm_direct.c
> > > @@ -101,6 +101,8 @@ retryget:
> > > if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
> > > IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
> > > first_instance = 1;
> > > + if (dmix->shmid < 0 && errno == EEXIST)
> > > + goto retryget;
> >
> > Hrm, but this would result in an endless loop if the shm was already
> > taken persistently.
>
> If so, shouldn't the first call to shmget succeed?
>
> To me it seems very unlikely that both calls continuosly fail.
Well, you are inserting a loop at the code below:
retryget:
dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
dmix->ipc_perm);
if (dmix->shmid < 0) {
if (errno == ENOENT)
if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
first_instance = 1;
==> if (dmix->shmid < 0 && errno == EEXIST)
==> goto retryget;
}
It's in the if block when the first shmget() fails. If there was
already a shm with the given id that had been assigned by another (not
necessarily by alsa-lib but by whatever program), the first shmget
returns an error with EEXIST. Then it goes back again in a loop; and
it can be endless if another program doesn't release the shm.
> > Also, which call does give a negative shmid, actually? It's from the
> > first shmget() or the second shmget()?
>
> What happens is that both threads go down that path but, of course,
> only one succeeds in the second shmget call.
This should have been protected by a sempahore beforehand.
And if it's about threads, the application itself has to take care of
the race. alsa-lib is no thread-safe, after all.
So, did you see the issue with multiple processes, or is it about the
multi-threads?
Takashi
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