On Saturday 14 March 2015 15:03:08 Josh Triplett wrote: > I had to think about this for a while, but I think it makes sense now. > wait should *not* ever return the PID of an autoreaped process, because > that would introduce a race condition (the caller cannot safely do > *anything* with the PID of an autoreaped process, since by the time it > does, the process may be gone and the PID may be reused). However, that > doesn't mean wait cannot block on the process, and then subsequently > wake up and return -ECHILD (or keep waiting on some other child process > if there is one). That's apparently the semantic used with SA_NOCLDWAIT > or if you have SIGCHLD set to SIG_IGN, and matching that seems > appropriate. > > Thiago, could your QProcess implementation handle that modified autoreap > semantic? The downside there is that if your calling process has a > process-wide loop that waits for all processes (and explicitly passes > the Linux-specific __WCLONE or __WALL flag, since your processes > launched with a 0 signal would count as "clone" children), they'd get > back the processes you launch, too. (That would happen with your > userspace-emulated version too for calls *without* __WCLONE or __WALL.) > You'd still get the exit status you need via the clonefd, without a > race, and you wouldn't need to touch process-wide signal handling, so I > think this should still work and avoid any races. I don't see why QProcess would have a problem. We don't have such a process- wide wait loop with __WCLONE or __WALL and I can't think of any reason why someone would do that and still expect NPTL to work. Or, put another way, if they are using clone/clone4 directly and bypassing NPTL, they're probably in a very specialised process that has no business running QProcess in the first place. I wouldn't be too worried. Inside glibc itself, __WCLONE is used only in unit tests and __WALL is used in a loop in elf/pldd.c, which is an independent application. Bionic has __WCLONE in tests only too. -- Thiago Macieira - thiago.macieira (AT) intel.com Software Architect - Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
