Date: Sat, 15 Feb 2020 12:27:26 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [timer] timer_delete function async problem On Sat, Feb 15, 2020 at 06:54:23PM +0800, zuotina wrote: > Hi everrone > > > Problem: > When I created SIGEV_THREAD timer, then start it by timer_settime. like this > the notify callback in the helper thread 'start' will be run when timer expiration. > But when I delete the timer, the notify callback will be run all the same. > This is not what i want. In actual use, I encountered a problem. > > > I found that the 'timer_delete' function returns immediately after called. > The timer may not perform the delete action. Logically the timer is deleted before the timer_delete function returns. If the handler thread is still running a handler at the time, that will necessarily continue intil it exits; the specification makes no provision for timer_delete doing anything to already-running handler threads. Since the operations are inherently unordered, it's possible that a new handler physically starts running after the call to timer_delete is made but before the signal is sent; as far as I can tell this is not observable by the application (since there is no ordering). > In addition, the SIGEV_SIGNAL timer can be deleted after called the function. > So i think the function has different semantics for different types. But doing so does not stop the signal handler from running (and fundamentally couldn't). So I don't understand what your concern is. Is it just that the kernel timer resource still exists until the handler thread finishes? I don't think that's visible to the application except possibly in limiting the number of timers that can be created. > Is there a way to implement synchronously ? At some point I plan to drop use of kernel timer resources entirely for SIGEV_THREAD timers, since they can be implemented *more easily*, with fewer hacks (no SIGTIMER at all! we can get rid of this reserved RT signal) with just a loop performing clock_nanosleep. If/when this change is made, there will be no kernel timer resource involed at all, so if I understand what you're asking, I guess that would give the behavior you want. (?) If I'm not understanding what you're asking for, could you send a minimal testcase program demonstrating how you observe a behavior you consider wrong without the test invoking any UB? Rich
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