On Sun, 30 Sep 2007, Denys Vlasenko wrote: > Hi Ulrich, > > On Friday 28 September 2007 18:34, Ulrich Drepper wrote: > > One more small change to extend the availability of creation of > > file descriptors with FD_CLOEXEC set. Adding a new command to > > fcntl() requires no new system call and the overall impact on > > code size if minimal. > > Tangential question: do you have any idea how userspace can > safely do nonblocking read or write on a potentially-shared fd? > > IIUC, currently it cannot be done without races: > > old_flags = fcntl(fd, F_GETFL); > ...other process may change flags!... > fcntl(fd, F_SETFL, old_flags | O_NONBLOCK); > read(fd, ...) > ...other process may see flags changed under its feet!... > fcntl(fd, F_SETFL, old_flags); > > Can this be fixed?
I'm not sure I understood correctly your use case. But, if you have two processes/threads randomly switching O_NONBLOCK on/off, your problems arise not only the F_SETFL time. If one of the tasks is not expecting an fd to be O_NONBLOCK, that will likely end up not handling correctly read/write-miss situations. In that case it'd be better to keep the fd as O_NONBLOCK, and manually create blocking behaviour (when needed) with poll+read/write. - Davide - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
