On February 11, 2019 15:57, Junio C Hamano wrote: > "Randall S. Becker" <rsbecker@xxxxxxxxxxxxx> writes: > > > Hi All, > > > > I found this while trying to track down a hang in t5562 - this isn't > > the fix, but here it is something that could be considered a > > code-inspection. If there have been random unexplained hangs when git > > runs as a daemon, this might be the cause. > > > > According to many systems (other than Linux), the revents field is > > supposed to be 0 on return to poll(). This was the cause of some > > heart-ache a while back in compat/poll/poll.c. > > I am having a hard time grokking "supposed to be 0 on return to", but do you > mean "the caller must clear .revents field before calling poll()"? > > http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html > has this > > In each pollfd structure, poll() shall clear the revents member, > except that where the application requested a report on a condition > by setting one of the bits of events listed above, poll() shall set > the corresponding bit in revents if the requested condition is > true. In addition, poll() shall set the POLLHUP, POLLERR, and > POLLNVAL flag in revents if the condition is true, even if the > application did not set the corresponding bit in events. > > and I am also having a hard time interpreting the "except that". If we asked > to report (e.g. we set POLLIN in the .events field), poll() does not have to > clear .revents but just set whatever bits it needs to set to report the > condition in the field? > > If that is the case, it makes it a bug not to clear .revents before calling poll; > the sample code snippet on the same page in EXAMPLES section does not, > though, so I am puzzled. > > In any case, no matter what POSIX says, if clearing .revents before calling > poll() helps on platforms in the real world, the patch is worth taking as a fix, I > would think. That's what my intent was - my explanations are suffering from a little work-induced sleep deprivation. Would you like this as a formal patch? > > > I am not certain whether that copy of poll() is used in daemon, but I > > wanted to point out that the value is being returned to poll, outside > > of compat/poll/poll.c and may present a potential for poll returning > > an error on that FD due to random values that might be in revents. > > > > Please see 61b2a1acaae for a related change/justification. > > > > diff --git a/daemon.c b/daemon.c > > index 9d2e0d20ef..1e275fc8b3 100644 > > --- a/daemon.c > > +++ b/daemon.c > > @@ -1194,6 +1194,7 @@ static int service_loop(struct socketlist *socklist) > > } > > handle(incoming, &ss.sa, sslen); > > } > > + pfd[i].revents = 0; > > } > > } > > } Regards, Randall
