Kamil, # Sorry for late response. On Fri, Dec 13, 2013 at 05:00:04PM -0600, Kamil Iskra wrote: > On Fri, Dec 13, 2013 at 14:59:02 -0500, Naoya Horiguchi wrote: > > Hi Naoya, > > > On Thu, Dec 12, 2013 at 04:25:27PM -0600, Kamil Iskra wrote: > > > Please find below a trivial patch that changes the sending of BUS_MCEERR_AO > > > SIGBUS signals so that they can be handled by an arbitrary thread of the > > > target process. The current implementation makes it impossible to create a > > > separate, dedicated thread to handle such errors, as the signal is always > > > sent to the main thread. > > This can be done in application side by letting the main thread create a > > dedicated thread for error handling, or by waking up existing/sleeping one. > > It might not be optimal in overhead, but note that an action optional error > > does not require to be handled ASAP. And we need only one process to handle > > an action optional error, so no need to send SIGBUS(BUS_MCEERR_AO) for every > > processes/threads. > > I'm not sure if I understand. "letting the main thread create a dedicated > thread for error handling" is exactly what I was trying to do -- the > problem is that SIGBUS(BUS_MCEERR_AO) signals are never sent to that > thread, which is contrary to common expectations. The signals are sent to > the main thread only, even if SIGBUS is masked there. I think that what your patch suggests is that "letting the dedicated thread get SIGBUS(BUS_MCEERR_AO) directly (not via the main thread) from kernel." It's a bit different from what I meant in the previous email. > Just to make sure that we're on the same page, here's a testcase that > demonstrates the problem I'm trying to fix (I should've sent it the first > time; sorry for being lazy): Thanks. And I see your problem. > > #include <pthread.h> > #include <signal.h> > #include <stdio.h> > #include <stdlib.h> > #include <sys/prctl.h> > #include <unistd.h> > > void sigbus_handler(int sig, siginfo_t* si, void* ucontext) > { > printf("SIGBUS caught by thread %ld, code %d, addr %p\n", > (long)pthread_self(), si->si_code, si->si_addr); > } > > void* sigbus_thread(void* arg) > { > printf("sigbus thread: %ld\n", (long)pthread_self()); > for (;;) > pause(); > } > > int main(void) > { > struct sigaction sa; > sigset_t mask; > char* buf; > pthread_t thread_id; > > prctl(PR_MCE_KILL, PR_MCE_KILL_SET, PR_MCE_KILL_EARLY, 0, 0); > > posix_memalign((void*)&buf, 4096, 4096); > buf[0] = 0; > printf("convenient address to hard offline: %p\n", buf); > > sa.sa_sigaction = sigbus_handler; > sigemptyset(&sa.sa_mask); > sa.sa_flags = SA_SIGINFO; > sigaction(SIGBUS, &sa, NULL); > > pthread_create(&thread_id, NULL, sigbus_thread, NULL); > > sigemptyset(&mask); > sigaddset(&mask, SIGBUS); > pthread_sigmask(SIG_BLOCK, &mask, NULL); > > printf("main thread: %ld\n", (long)pthread_self()); > > for (;;) > pause(); > > return 0; > } > > > This testcase uses a very common signal handling strategy in multithreaded > programs: masking signals in all threads but one, created specifically for > signal handling. It works just fine if I send it SIGBUS from another > terminal using "kill". It does not work if I offline the page: the signal > is routed to the main thread, where it's marked as pending; nothing gets > printed out. > > As you were so kind to point out, SIGBUS(BUS_MCEERR_AO) does not need to be > handled ASAP, so why should the kernel handle it differently to other > non-critical signals? The current behavior seems inconsistent, and there > is no convenient workaround (as a library writer, I have no control over > the actions of the main thread). I'm not sure if current implementation is intentional or not, but I understand about the inconsistency. > > And another concern is if this change can affect/break existing applications. > > If it can, maybe you need to add (for example) a prctl attribute to show that > > the process expects kernel to send SIGBUS(BUS_MCEERR_AO) only to the main > > thread, or to all threads belonging to the process. > > I understand your concern. However, I believe that having > SIGBUS(BUS_MCEERR_AO) behave consistently with established POSIX standards > for signal handling outhweighs the concerns over potential > incompatibilities, especially with a feature that is currently used by a > very small subset of applications. OK, and in this case the effect on existing multi-threaded applications seems to be small (just small degradation of availability, but no kernel panic nor data lost,) so I think it's acceptable. I want to agree with your patch, so could you repost the patch with patch description? git-format-patch will help you. Thanks, Naoya Horiguchi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>