Messages in this thread | | | Date | Wed, 5 Sep 2007 16:45:45 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: [PATCH] Revised timerfd() interface |
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On Thu, 6 Sep 2007, Michael Kerrisk wrote:
> Hi Davide, > > > > > > > > As I think about this more, I see more problems with > > > > > > > your argument. timerfd needs the ability to get and > > > > > > > get-while-setting just as much as the earlier APIs. > > > > > > > Consider a library that creates a timerfd file descriptor that > > > > > > > is handed off to an application: that library may want > > > > > > > to modify the timer settings without having to create a > > > > > > > new file descriptor (the app mey not be able to be told about > > > > > > > the new fd). Your argument just doesn't hold, AFAICS. > > > > > > > > > > > > Such hypotethical library, in case it really wanted to offer such > > > > > > functionality, could simply return an handle instead of the raw > > > > > > fd, and take care of all that stuff in userspace. > > > > > > > > > > Did I miss something? Is it not the case that as soon as the > > > > > library returns a handle, rather than an fd, then the whole > > > > > advantage of timerfd() (being able to select/poll/epoll on > > > > > the timer as well as other fds) is lost? > > > > > > > > Why? The handle would simply be a little struct where the timerfd fd > > > > is > > > > stored, and a XXX_getfd() would return it. > > > > So my point is, I doubt such functionalities are really needed, and I > > > > also argue that the kernel is the best place for such wrapper code > > > > to go. > > > > > > So what happens if one thread (via the library) wants modify > > > a timer's settings at the same timer as another thread is > > > select()ing on it? The first thread can't do this by creating > > > a new timerfd timer, since it wants to affect the select() > > > in the other thread? > > > > It can be done w/out any problems. The select thread will be notified > > whenever the new timer setting expires. > > We are going in circles here. I think you are missing my point. > Consider the following > > [[ > Thread A: calls library function which creates a timerfd file > descriptor. > > Thread B: calls select() on the timerfd file descriptor. > > Thread A: calls library function which wants to: > a) modify timer settings, and retrieve copy of current timer > settings, and later > b) restore old timer settings. > ]] > > This seems a quite reasonable use-case to me, and the existing > interface simply can't support it.
"Quite reasonable"? :) I honestly doubt it, but anyway. Modulo error checking:
struct tfd { int fd, clockid; struct itimerspec ts; };
struct tfd *tfd_create(int clockid, int flags, const struct itimerspec *ts) { struct tfd *th; th = malloc(sizeof(*th)); th->clockid = clockid; th->ts = *ts; th->fd = timerfd(-1, clockid, flags, ts); return th; }
void tfd_close(struct tfd *th) { close(th->fd); free(th); }
int tfd_getfd(const struct tfd *th) { return th->fd; }
int tfd_gettime(const struct tfd *th, int *clockid, struct itimerspec *ts) { *clockid = th->clockid; *ts = th->ts; return 0; }
int tfd_settime(struct tfd *th, int clockid, int flags, const struct itimerspec *ts) { th->fd = timerfd(th->fd, clockid, flags, ts); th->clockid = clockid; th->ts = *ts; return 0; }
Wrap the get/set with a mutex in case you plan to shoot yourself in a foot by doing get/set from multiple threads ;) So, once again:
- I sincerly doubt the above is common usage/design patters for timerfds
* timerfds are not a common global resource, ala signals, that requires get+set+restore pattern - you can have many of them set to different times
- Those IMO *very* special use cases can be handled in userspace with few lines of code, *if* really needed
- Davide
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