Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes: > While trying to address the longstanding FIXME in the posix timer code > related to ignored signals, I stumbled over the following issue: > > I blocked the signal of the timer, then installed the SIG_IGN handler, > created and started the timer. After a short sleep the timer has fired > several times, but it's still ignored AND blocked. > > Calling sigpending() after that has the timer signal set. See test case > below. > > But 'man sigpending' says: > > "If a signal is both blocked and has a disposition of "ignored", it is _not_ > added to the mask of pending signals when generated." > > So something is clearly wrong here. > > The same happens with sigwait() while the signal is still blocked and > ignored, it returns with that signal number and has the signal dequeued. > > > The whole blocked vs. ignored handling is inconsistent both in the posix > spec and in the kernel. > > The only thing vs. ignored signals what the spec mandates is: > > SIG_IGN: > > Delivery of the signal shall have no effect on the process. > > ... > > Setting a signal action to SIG_IGN for a signal that is pending shall > cause the pending signal to be discarded, whether or not it is blocked. > > ... > > Any queued values pending shall be discarded and the resources used to > queue them shall be released and made available to queue other signals. > > That's exactly what the kernel does in do_sigaction(). > > And for everything else the spec is blurry: > > If the action associated with a blocked signal is to ignore the signal > and if that signal is generated for the process, it is unspecified > whether the signal is discarded immediately upon generation or remains > pending. > > So the kernel has chosen to keep them pending for whatever reasons, which > does not make any sense to me, but there is probably a historic reason. > > The commit which added the queuing of blocked and ignored signals is in the > history tree with a pretty useless changelog. > > https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git > > commit 98fc8ab9e74389e0c7001052597f61336dc62833 > Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxxx> > Date: Tue Feb 11 20:49:03 2003 -0800 > > Don't wake up processes unnecessarily for ignored signals > > It rewrites sig_ignored() and adds the following to it: > > + /* > + * Blocked signals are never ignored, since the > + * signal handler may change by the time it is > + * unblocked. > + */ > + if (sigismember(&t->blocked, sig)) > + return 0; > > I have no idea how that is related to $subject of the commit and why this > decision was made. > > Linus, any recollection? > > IMO, it's perfectly reasonable to discard ignored signals even when the > signal is in the blocked mask. When its unblocked and SIG_IGN is replaced > then the next signal will be delivered. But hell knows, how much user space > depends on this weird behaviour by now. I just looked through the history and the commit you point to looks like it was either code motion or a regression fix. The change to ignore blocked signals actually came in between 1.2 and 2.0. It looks like the relevant diff was: commit 886bad3fe1fe0c67208f15a02047e450e30f2b3a Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Date: Mon Apr 1 16:00:00 1996 -0800 Linux version 1.3.82 diff --git a/kernel/exit.c b/kernel/exit.c index 2b8e6d13ba1f..329d0b36bb08 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -23,25 +23,28 @@ extern void kerneld_exit(void); int getrusage(struct task_struct *, int, struct rusage *); -static int generate(unsigned long sig, struct task_struct * p) +static inline void generate(unsigned long sig, struct task_struct * p) { unsigned long mask = 1 << (sig-1); struct sigaction * sa = sig + p->sig->action - 1; - /* always generate signals for traced processes ??? */ - if (!(p->flags & PF_PTRACED)) { + /* + * Optimize away the signal, if it's a signal that can + * be handled immediately (ie non-blocked and untraced) + * and that is ignored (either explicitly or by default) + */ + if (!(mask & p->blocked) && !(p->flags & PF_PTRACED)) { /* don't bother with ignored signals (but SIGCHLD is special) */ if (sa->sa_handler == SIG_IGN && sig != SIGCHLD) - return 0; + return; /* some signals are ignored by default.. (but SIGCONT already did its deed) */ if ((sa->sa_handler == SIG_DFL) && (sig == SIGCONT || sig == SIGCHLD || sig == SIGWINCH || sig == SIGURG)) - return 0; + return; } p->signal |= mask; if (p->state == TASK_INTERRUPTIBLE && (p->signal & ~p->blocked)) wake_up_process(p); - return 1; } int send_sig(unsigned long sig,struct task_struct * p,int priv) Eric -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
