linux-kernel - Re: [BUG on PREEMPT_RT, 2.6.23.1-rt5] in rt-mutex code and signals

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Date:	Sat, 17 Nov 2007 18:09:15 +0100
From:	"Remy Bohmer" <linux@...mer.net>
To:	"Daniel Walker" <dwalker@...sta.com>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	RT <linux-rt-users@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG on PREEMPT_RT, 2.6.23.1-rt5] in rt-mutex code and signals

Hello Daniel,

Thanks for looking into it also.
Steven already made clear to me that the 'struct semaphore' type on
the RT-kernel should not be used as a counting-semaphore, but as some
sort of legacy-mutex... (The confusion that this will cause is clear
by now...)

I still do not understand the problems I had with the
interruptible-waits on a real rt-mutex, but I have to figure that out
again on Monday. Maybe one confusion let to another...

(Note, A completion will not work for me, because they are not
designed for reuse across several threads. The read/write runs in user
context and as such it can be called by different threads, which would
require a init of a completion before waiting on it, but that would be
racy, I could miss the awake by the init)

> So I converted your code to use a compat_semaphore, and no oops
> happens.. Which makes sense because compat_semaphores are designed to
> work the way your using them.

Actually, IMO, compat_semaphores behave like semaphores should behave,
and thus the same as they behave on a non-RT kernel, and at the
locations where the semaphores are now misused as mutexes on RT, we
should replace them by differently-named-mutex-type-semaphores, or
better: real-RT-mutexes..
IMO this wrong usage of semaphores is solved by modifying the code
that actually made proper use of the semaphores, and I think that if
the naming matches the mainline kernel, we only need to patch the
files that really need to be patched during the integration in
mainline of the RT-patch.

Kind Regards,

Remy Bohmer
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