Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries

From: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
	Darren Hart <dvhltc@us.ibm.com>,
	Gautham R Shenoy <ego@in.ibm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linuxppc-dev@ozlabs.org, Will Schmidt <willschm@us.ibm.com>,
	Paul Mackerras <paulus@samba.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries
Date: Fri, 23 Jul 2010 12:37:01 +0530	[thread overview]
Message-ID: <20100723070701.GC26162@dirshya.in.ibm.com> (raw)
In-Reply-To: <1279861860.1970.74.camel@pasglop>

* Benjamin Herrenschmidt <benh@kernel.crashing.org> [2010-07-23 15:11:00]:

> On Fri, 2010-07-23 at 10:38 +0530, Vaidyanathan Srinivasan wrote:
> > Yes.  extended_cede_processor() will return with interrupts enabled in
> > the cpu. (This is done by the hypervisor).  Under normal cases we
> > cannot be interrupted because no IO interrupts are routed to us after
> > xics_teardown_cpu() and since the CPU is out of the map, nobody will
> > send us IPIs.
> 
> What about decrementer ?

Decrementer expiry event handling is bit complex.  The event as such
may not bring back the extended_cede_processor() cpu, but may be
marked pending when we get out of this state eventually.  I will find
more information on this event and update.

> > Though H_CEDE will return with interrupts enabled, it is unlikely that
> > an interrupt can be delivered in this context. 
> 
> Well, if interrupts are soft-disabled, even if one occurs, we will just
> mask and return, so that at least should be ok.

Yes.  We will immediately return to the extended_cede_processor() in
the while loop until the preferred_offline_state is changed.

--Vaidy