On Fri, Oct 14, 2022 at 08:01:29AM -0400, Elad Lahav wrote:
> Hello,
>
> I'm not sure about the statement made in this section: "In contrast,
> Linux avoids these hints, in-
> stead getting similar results from a mechanism called futexes".
> Futexes are just wrappers around kernel mutexes, to speed up the case
> of acquiring a free lock. As such they do not solve the priority
> inversion problem, and are certainly not a replacement for disabling
> pre-emption when using spin locks.
> Kernel-level mutexes may address priority inversion by employing one
> of the established protocols (priority inheritance or priority
> protection).
>
> Am I missing something?
Futexes were the response of the Linux kernel community to the request
from certain large database vendors for the ability to disable preemption
in userspace applications. I was there and I heard it with my own ears,
but to your point I find little on the web to back this up. In fact,
I am not easily finding the fact that most proprietary UNIX kernels did
in fact have userspace disable-preemption hints.
Futexes are more a sleep/wakeup mechanism than a wrapper around mutexes,
though there was a hilarious bug early on that allowed a usermode
application to force a kernel-mode deadlock. The semantics are involved,
a fact being driven home to those attempting to standardize it.
How about the following change? This documents futexes as the response
to the request for scheduler-conscious synchronization, but does not
claim anything beyond that.
Thanx, Paul
------------------------------------------------------------------------
commit 63c4deb868d5eec9df96fd8810e58e1519abb392
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date: Wed Oct 19 15:14:03 2022 -0700
locking: Avoid over-claiming for futexes
It is true that futexes were the response of the Linux-kernel community
to requests for user-mode disabling of preemption, but it is not clear
how futexes were evaluated by those making the requests.
Reported-by: Elad Lahav <e2lahav@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/locking/locking.tex b/locking/locking.tex
index b138cd59..6d3bb1c8 100644
--- a/locking/locking.tex
+++ b/locking/locking.tex
@@ -2057,8 +2057,9 @@ being placed in a machine register.
These hints frequently take the form of a bit set in a particular
machine register, which enables extremely low per-lock-acquisition overhead
for these mechanisms.
-In contrast, Linux avoids these hints, instead getting
-similar results from a mechanism called
+In contrast, Linux avoids these hints.
+Instead, the Linux kernel community's response to requests for
+scheduler-conscious synchronization was a mechanism called
\emph{futexes}~\cite{HubertusFrancke2002Futex,IngoMolnar2006RobustFutexes,StevenRostedt2006piFutexes,UlrichDrepper2011Futexes}.
Interestingly enough, atomic instructions are not strictly needed to
