On Tue, 2007-06-05 at 17:37 -0700, Davide Libenzi wrote: > On Tue, 5 Jun 2007, Nicholas Miell wrote: > > > On Tue, 2007-06-05 at 17:11 -0700, Davide Libenzi wrote: > > > On Tue, 5 Jun 2007, Nicholas Miell wrote: > > > > > > > Yes, that's certainly wrong, but that's an implementation issue. I was > > > > more concerned about the design of the API. > > > > > > > > Naively, I would expect a reads on a signalfd to return either process > > > > signals or thread signals targeted towards the thread doing the read. > > > > > > > > What it actually does (delivering process signals or thread signals > > > > targeted towards the thread that created the signalfd) is weird. > > > > > > > > For one, it means you can't create a single signalfd, stick it in an > > > > epoll set, and then wait on that set from multiple threads. > > > > > > In your box threads do share the sighand, don't they? :) > > > > > > > I have no idea what you're trying to say, but it doesn't appear to > > address the issue I raise. > > "For one, it means you can't create a single signalfd, stick it in an > epoll set, and then wait on that set from multiple threads." > > Why not? > A signalfd, like I said, is attached to the sighand, that is shared by the > threads. > >
POSIX requires the following: "At the time of generation, a determination shall be made whether the signal has been generated for the process or for a specific thread within the process. Signals which are generated by some action attributable to a particular thread, such as a hardware fault, shall be generated for the thread that caused the signal to be generated. Signals that are generated in association with a process ID or process group ID or an asynchronous event, such as terminal activity, shall be generated for the process." In practice, this means that signals like SIGSEGV/SIGFPE/SIGILL/etc. and signals generated by pthread_kill() (i.e. tkill() or tgkill()) are directed to a specific threads, while other signals are directed to the process as a whole and serviced by any thread that isn't blocking that specific signal. Linux accomplishes this by having two lists of pending signals -- current->pending is the per-thread list and current->signal->shared_pending is the process-wide list. dequeue_signal(tsk, ...) looks for signals first in tsk->pending and then in tsk->signal->shared_pending. sys_signalfd() stores current in signalfd_ctx. signalfd_read() passes that context to signalfd_dequeue, which passes that that saved task_struct pointer to dequeue_signal. This means that a signalfd will deliver signals targeted towards either the original thread that created that signalfd, or signals targeted towards the process as a whole. This means that a single signalfd is not adequate to handle signal delivery for all threads in a process, because signals targeted towards threads other than the thread that originally created the signalfd will never be queued to that signalfd. Is my analysis wrong? -- Nicholas Miell <[EMAIL PROTECTED]> - 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/
