On Fri, Sep 20, 2019 at 10:28:36AM -0700, William Tu wrote: > On Wed, Sep 18, 2019 at 04:27:46PM -0700, Ben Pfaff wrote: > > On Wed, Sep 18, 2019 at 01:16:37PM -0700, William Tu wrote: > > > Thanks for the feedback. > > > > > > On Wed, Sep 18, 2019 at 11:30 AM Ben Pfaff <b...@ovn.org> wrote: > > > > > > > > On Tue, Sep 17, 2019 at 04:13:24PM -0700, William Tu wrote: > > > > > The patch catches the SIGSEGV signal and prints the backtrace > > > > > using libunwind, hopefully makes it easier to debug. > > > > > > > > > > Signed-off-by: William Tu <u9012...@gmail.com> > > > > > > > > I guess my experience is that this sort of thing is sometimes useful but > > > > ultimately causes problems because it tries to call a lot of functions > > > > that a signal handler is not supposed to call. > > > > > > Do you mean the show_backtrace() tries to call too many functions? > > > What's the concern about calling lots of functions in a signal handler? > > > > There is no concern about the number of functions but about the specific > > ones. Most C library functions are not async-signal-safe, meaning that > > it is not safe to call them from a signal handler. There is a list of > > functions guaranteed to be async-signal-safe in section 2.4.3 "Signal > > Actions" of the System Interfaces volume of POSIX. You can find it > > here: https://pubs.opengroup.org/onlinepubs/9699919799/. Notably, it > > doesn't contain the stdio functions like printf() and fflush(), or > > syslog, and those functions are likely to malfunction if the signal > > handler interrupts some ongoing operation on a stream. There's a risk > > of being caught in some kind of SIGSEGV loop. > > > > I think there needs to be a lot of care if we're going to do this by > > default. > > Thanks a lot! I wasn't aware of these issue. > > In lib/backtrace.c, it uses GNU backtrace(3) and it's not signal safe. > Fortunately, the libunwind in this patch with local unwinding is signal > safe, > https://www.nongnu.org/libunwind/man/libunwind(3).html#section_5 > So for v2, I'm thinking about removing the use of printf and fllush.
That sounds like a good idea. It is still possible to write to stderr using write(). It might be possible to define some signal-safe vlog interface for logging to a file, too. _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev
