Messages in this thread |  | | Date | Tue, 19 Nov 2013 10:44:38 +1100 | From | NeilBrown <> | Subject | Re: [PATCH/RFC] wait_for_completion_timeout() considered harmful. |
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On Mon, 18 Nov 2013 15:27:46 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:
> On Sun, 17 Nov 2013 08:06:03 +1100 NeilBrown <neilb@suse.de> wrote: > > > It would be reasonable to assume that > > > > wait_for_completion_timeout(&wm8350->auxadc_done, msecs_to_jiffies(5)); > > > > would wait at least 5 msecs for the auxadc_done to complete. But it does not. > > With a HZ of 200 or less, msecs_to_jiffies(5) has value '1', and so this > > will only wait until the next "timer tick", which could happen immediately. > > > > This can lead to incorrect results - and has done so in out-of-tree patches > > for drivers/misc/bmp085.c which uses a very similar construct to enable interrupt > > based result collection. > > > > The documentation for several *_timeout* functions claim they will wait for > > "timeout jiffies" to have elapsed where this is not the case. They will > > actually wait for "timeout" jiffies to have started implying an elapsed time > > between (timeout-1) and (timeout). > > > > This patch corrects some of this documentation, and adds a collection of > > wait_for_completion*_msecs() > > interfaces which wait at least the given number of milliseconds for the > > completion (or a signal). > > Mutter. wait_for_x(..., 5ms) should wait for a minimum of 5ms, no matter > what. > > So I'd suggest we make that happen, rather than adding some new interfaces?
I thought of that. It would certainly be nice.
However what we have is XXX_timeout(...., jiffies). And if we decided that XXX_timeout(...., msecs_to_jiffies(5)) would only timeout after at least 5ms, then schedule_timeout(1) would have to wait at least one full jiffie, which is quite different to what it currently does.
We have loops that have timeout = schedule_timeout(timeout) in the middle and if we change the semantics of schedule_timeout() to round up, those loops could wait quite a bit longer than expected.
So I think that we do need to add new interfaces just like msleep() was introduced a while back to fix all the various misuses of schedule_timeout(msecs_to_jiffies(XX))).
Possibly we can also discard old bad interfaces. Maybe the *_timeout() interfaces should become *_until() where the jiffies number isn't a count but is a value that we wait for "jiffies" to exceed.
I don't think there is a really easy solution, but thanks for pushing the discussion along towards trying to understand one.
NeilBrown
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