Re: [PATCH] __check_pf: Add a cancellation cleanup handler [BZ #20975]

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Apr 28, 2023 at 12:17 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Thu, Apr 27, 2023 at 3:06 PM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > There are reports for hang in __check_pf:
> >
> > https://github.com/JoeDog/siege/issues/4
> >
> > It is reproducible only under specific configurations:
> >
> > 1. Large number of cores (>= 64) and large number of threads (> 3X of
> > the number of cores) with long lived socket connection.
> > 2. Low power (frequency) mode.
> > 3. Power management is enabled.
> >
> > While holding lock, __check_pf calls make_request which calls __sendto
> > and __recvmsg.  Since __sendto and __recvmsg are cancellation points,
> > lock held by __check_pf won't be released and can cause deadlock when
> > thread cancellation happens in __sendto or __recvmsg.  Add a cancellation
> > cleanup handler for __check_pf to unlock the lock when cancelled by
> > another thread.  This fixes BZ #20975 and the siege hang issue.
> > ---
> >  sysdeps/unix/sysv/linux/Makefile   |  2 ++
> >  sysdeps/unix/sysv/linux/check_pf.c | 15 +++++++++++++++
> >  2 files changed, 17 insertions(+)
> >
> > diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> > index aec7a94785..0160be8790 100644
> > --- a/sysdeps/unix/sysv/linux/Makefile
> > +++ b/sysdeps/unix/sysv/linux/Makefile
> > @@ -529,6 +529,8 @@ sysdep_headers += \
> >  sysdep_routines += \
> >    netlink_assert_response \
> >    # sysdep_routines
> > +
> > +CFLAGS-check_pf.c += -fexceptions
> >  endif
> >
> >  # Don't compile the ctype glue code, since there is no old non-GNU C library.
> > diff --git a/sysdeps/unix/sysv/linux/check_pf.c b/sysdeps/unix/sysv/linux/check_pf.c
> > index b157c5126c..2b0b8b6368 100644
> > --- a/sysdeps/unix/sysv/linux/check_pf.c
> > +++ b/sysdeps/unix/sysv/linux/check_pf.c
> > @@ -292,6 +292,14 @@ make_request (int fd, pid_t pid)
> >    return NULL;
> >  }
> >
> > +#ifdef __EXCEPTIONS
> > +static void
> > +cancel_handler (void *arg __attribute__((unused)))
> > +{
> > +  /* Release the lock.  */
> > +  __libc_lock_unlock (lock);
> > +}
> > +#endif
> >
> >  void
> >  attribute_hidden
> > @@ -304,6 +312,10 @@ __check_pf (bool *seen_ipv4, bool *seen_ipv6,
> >    struct cached_data *olddata = NULL;
> >    struct cached_data *data = NULL;
> >
> > +#ifdef __EXCEPTIONS
> > +  /* Make sure that lock is released when the thread is cancelled.  */
> > +  __libc_cleanup_push (cancel_handler, NULL);
> > +#endif
> >    __libc_lock_lock (lock);
> >
> >    if (cache_valid_p ())
> > @@ -338,6 +350,9 @@ __check_pf (bool *seen_ipv4, bool *seen_ipv6,
> >         }
> >      }
> >
> > +#ifdef __EXCEPTIONS
> > +  __libc_cleanup_pop (0);
> > +#endif
> >    __libc_lock_unlock (lock);
> >
> >    if (data != NULL)
> > --
> > 2.40.0
> >
>
> LGTM.

OK for release branches?

Thanks.

-- 
H.J.