Disambiguate condvar API contract

On Illumos callers of cv_timedwait and cv_timedwait_hires
can't distinguish between whether or not the cv was signaled
or the call timed out. Illumos handles this (for some definition
of handles) by calling cv_signal in the return path if we were
signaled but the return value indicates instead that we timed
out. This would make sense if it were possible to query the the
cv for its net signal disposition. However, this isn't possible
and, in spite of the fact that there are places in the code that
clearly take a different and incompatible path if a timeout value
is indicated, this distinction appears to be rather subtle to most
developers. This problem is further compounded by the fact that on
Linux, calling cv_signal in the return path wouldn't even do the
right thing unless there are other waiters.

Since it is possible for the caller to independently determine how
much time is remaining but it is not possible to query if the cv
was in fact signaled, prioritizing signalling over timeout seems
like a cleaner solution. In addition, judging from usage patterns
within the code itself, it is also less error prone.

Reviewed-by: Jorgen Lundman <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Matt Macy <[email protected]>
Closes #10471 

1 files changed, 2 insertions, 2 deletions

diff --git a/include/sys/zfs_context.h b/include/sys/zfs_context.h
index 72fcd74ca..7d5567db4 100644
--- a/include/sys/zfs_context.h
+++ b/include/sys/zfs_context.h
@@ -315,8 +315,8 @@ extern void cv_init(kcondvar_t *cv, char *name, int type, void *arg);
 extern void cv_destroy(kcondvar_t *cv);
 extern void cv_wait(kcondvar_t *cv, kmutex_t *mp);
 extern int cv_wait_sig(kcondvar_t *cv, kmutex_t *mp);
-extern clock_t cv_timedwait(kcondvar_t *cv, kmutex_t *mp, clock_t abstime);
-extern clock_t cv_timedwait_hires(kcondvar_t *cvp, kmutex_t *mp, hrtime_t tim,
+extern int cv_timedwait(kcondvar_t *cv, kmutex_t *mp, clock_t abstime);
+extern int cv_timedwait_hires(kcondvar_t *cvp, kmutex_t *mp, hrtime_t tim,
     hrtime_t res, int flag);
 extern void cv_signal(kcondvar_t *cv);
 extern void cv_broadcast(kcondvar_t *cv);