Messages in this thread | | | Date | Mon, 24 Oct 2011 10:04:08 +1100 | From | NeilBrown <> | Subject | Re: lsusd - The Linux SUSpend Daemon |
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On Sun, 23 Oct 2011 14:48:22 +0200 "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
> On Sunday, October 23, 2011, NeilBrown wrote: > > On Fri, 21 Oct 2011 22:00:13 -0400 (EDT) Alan Stern > > <stern@rowland.harvard.edu> wrote: > > > > > On Sat, 22 Oct 2011, NeilBrown wrote: > > > > > > > > > It uses files in /var/run/suspend for all communication. > > > > > > > > > > I'm not so keen on using files for communication. At best, they are > > > > > rather awkward for two-way messaging. If you really want to use them, > > > > > then at least put them on a non-backed filesystem, like something under > > > > > /dev. > > > > > > > > Isn't /var/run a tmpfs filesystem? It should be. > > > > Surely /run is, so in the new world order the files should probably go > > > > there. But that is just a detail. > > > > > > On my Fedora-14 systems there is no /run, and /var/run is a regular > > > directory in a regular filesystem. > > > > > > > I like files... I particularly like 'flock' to block suspend. The > > > > rest.... whatever.. > > > > With files, you only need a context switch when there is real communication. > > > > With sockets, every message sent must be read so there will be a context > > > > switch. > > > > > > > > Maybe we could do something with futexes... > > > > > > Not easily -- as far as I can tell, futexes enjoy relatively little > > > support. In any case, they provide the same service as a mutex, which > > > means you'd have to build a shared lock on top of them. > > > > > > > > > lsusd does not try to be event-loop based because: > > > > > > - /sys/power/wakeup_count is not pollable. This could probably be > > > > > > 'fixed' but I want code to work with today's kernel. It will probably > > > > > > > > > > Why does this matter? > > > > > > > > In my mind an event based program should never block. Every action should be > > > > non-blocking and only taken when 'poll' says it can. > > > > Reading /sys/power/wakeup_count can be read non-blocking, but you cannot find > > > > out when it is sensible to try to read it again. So it doesn't fit. > > > > > > There shouldn't be any trouble about making wakeup_count pollable. It > > > also would need to respect nonblocking reads, which it currently does > > > not do. > > > > Hmm.. you are correct. I wonder why I thought it did support non-blocking > > reads... > > I guess it was the code for handling an interrupted system call. > > > > I feel a bit uncomfortable with the idea of sysfs files that block but I > > don't think I can convincingly argue against it. > > A non-blocking flag could be passed in, but it would be a very messy change - > > lots of function call signatures changing needlessly: we would need a flag > > to the 'show' method ... or add a 'show_nonblock' method which would also be > > ugly. > > > > > > But I think there is a need to block - if there is an in-progress event then > > it must be possible to wait for it to complete as it may not be visible to > > userspace until then. > > We could easily enable 'poll' for wakeup_count and then make it always > > non-blocking, but I'm not really sure I want to require programs to use poll, > > only to allow them. And without using poll there is no way to wait. > > > > As wakeup_count really has to be single-access we could possibly fudge > > something by remembering the last value read (like we remember the last value > > written). > > > > - if the current count is different from the last value read, then return > > it even if there are in-progress events. > > - if the current count is the same as the last value read, then block until > > there are no in-progress events and return the new value. > > - enable sysfs_poll on wakeup_count by calling sysfs_notify_dirent at the > > end of wakeup_source_deactivated .... or calling something in > > kernel/power/main.c which calls that. However we would need to make > > sysfs_notify_dirent a lot lighter weight first. Maybe I should do that. > > > > Then a process that uses 'poll' could avoid reading wakeup_count except when > > it has changed, and then it won't block. And a process that doesn't use poll > > can block by simply reading twice - either explicitly or by going around a > > read then write and it fails > > loop a second time. > > > > I'm not sure I'm completely comfortable with that, but it is the best I could > > come up with. > > Well, you're now considering doing more and more changes to the kernel > just to be able to implement something in user space to avoid making > some _other_ changes to the kernel. That doesn't sound right to me.
:-) I thought I might get challenged on something like that.
I think the cases are different though.
I'm not presenting this code as a new feature. I don't need new features - I have user-space code which works correctly with the current kernel features.
However the precise usage of wakeup_count is a little unusual in that it blocks when you read. That doesn't mean that it cannot be used correctly, but it might limit the options available to a user-space program which wants to use it. I was just looking at ways to generalise the existing interface so that it matches the rest of the kernel better. I see it much more as a bug fix than as a new feature.
I'm not saying we need this patch, and I'm not even sure I like it. I just presented it as part of exploring exactly how the wakeup_count interface really works. It is an interface that I like and that does allow the original suspend-race problem to be solved, but that does not mean it is necessarily perfect.
Thanks, NeilBrown
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