On Thu, Jun 23, 2016 at 12:55:15PM -0700, Darren Hart wrote:
> On Thu, Jun 23, 2016 at 08:41:09PM +0200, Michael Kerrisk (man-pages) wrote:
> > On 06/23/2016 08:28 PM, Darren Hart wrote:
> > > On Thu, Jun 23, 2016 at 07:26:52PM +0200, Thomas Gleixner wrote:
> > > > On Thu, 23 Jun 2016, Darren Hart wrote:
> > > > > On Thu, Jun 23, 2016 at 03:40:36PM +0200, Thomas Gleixner wrote:
> > > > > In my opinion, we should treat the timeout value as relative for
> > > > > FUTEX_WAIT
> > > > > regardless of the CLOCK used.
> > > >
> > > > Which requires even more changes as you have to select which clock you
> > > > are
> > > > using for adding the base time.
> > >
> > > Right, something like the following?
> > >
> > >
> > > diff --git a/kernel/futex.c b/kernel/futex.c
> > > index 33664f7..c39d807 100644
> > > --- a/kernel/futex.c
> > > +++ b/kernel/futex.c
> > > @@ -3230,8 +3230,12 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int,
> > > op, u32, val,
> > > return -EINVAL;
> > >
> > > t = timespec_to_ktime(ts);
> > > - if (cmd == FUTEX_WAIT)
> > > - t = ktime_add_safe(ktime_get(), t);
> > > + if (cmd == FUTEX_WAIT) {
> > > + if (cmd & FUTEX_CLOCK_REALTIME)
> > > + t = ktime_add_safe(ktime_get_real(), t);
> > > + else
> > > + t = ktime_add_safe(ktime_get(), t);
> > > + }
> > > tp = &t;
> > > }
> > > /*
> >
> > Just in the interests of readability/maintainability, might it not
> > make some sense to recode the timeout handling for FUTEX_WAIT
> > within futex_wait(). I think that part of the reason we're in this
> > mess of inconsistency is that timeout interpretation is being handled
> > at too many different points in the code.
>
>
> I agree, that is indeed why I missed it in my original patch.
Or perhaps in do_futex() which is where the majority of the argument
interpretation is done, and which already has a switch statement for all op
codes. Maybe something like this:
diff --git a/kernel/futex.c b/kernel/futex.c
index 33664f7..c666715 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3157,6 +3157,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t
*timeout,
{
int cmd = op & FUTEX_CMD_MASK;
unsigned int flags = 0;
+ ktime_t t;
if (!(op & FUTEX_PRIVATE_FLAG))
flags |= FLAGS_SHARED;
@@ -3181,6 +3182,18 @@ long do_futex(u32 __user *uaddr, int op, u32 val,
ktime_t *timeout,
switch (cmd) {
case FUTEX_WAIT:
val3 = FUTEX_BITSET_MATCH_ANY;
+ /*
+ * The user-facing FUTEX_WAIT op interface receives a relative
+ * timeout. The kernel-side futex_wait() function accepts an
+ * absolute timeout. Convert the relative timeout to absolute.
+ */
+ if (timeout) {
+ if (op & FUTEX_CLOCK_REALTIME)
+ t = ktime_add_safe(ktime_get_real(), *timeout);
+ else
+ t = ktime_add_safe(ktime_get(), *timeout);
+ timeout = &t;
+ }
case FUTEX_WAIT_BITSET:
return futex_wait(uaddr, flags, val, timeout, val3);
case FUTEX_WAKE:
@@ -3230,8 +3243,6 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32,
val,
return -EINVAL;
t = timespec_to_ktime(ts);
- if (cmd == FUTEX_WAIT)
- t = ktime_add_safe(ktime_get(), t);
tp = &t;
}
/*
--
Darren Hart
Intel Open Source Technology Center
