From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephane Eranian Date: Wed, 09 Nov 2005 22:31:33 +0000 Subject: Re: Question about interrupt enabling/disabling in kernel exit path Message-Id: <20051109223133.GD4379@frankl.hpl.hp.com> List-Id: References: <1131559925.5214.100.camel@localhost.localdomain> In-Reply-To: <1131559925.5214.100.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org Lee, As you may have seen there is a perfmon callback from do_notify_resume(). I am also using TIF_NOTIFY_RESUME to get back into perfmon just before leaving the kernel. It is not true that interrupts are always off when getting there. Last year we ran into a issue for this reason, i.e., I was assume interrupts were always masked when getting there. But apparentlyy, that is not always the case. I don't recall exactly what is the path to get to this situation but I know it exists. On Wed, Nov 09, 2005 at 01:12:05PM -0500, Lee Schermerhorn wrote: > Hello: > > I have a question about enabling interrupts in the kernel exit path. > Especially, in do_notify_resume_user(). I have work that I need to > defer to this point, but it needs to run with interrupts enabled. I > wanted to piggy back off TIF_NOTIFY_RESUME, because that is more or less > generic code and gets handled in a C function where I can play the usual > header games: static inline wrapper function when feature is > configured; null macro when not. > > I see a comment at the beginning of ia64_leave_kernel that says that > "work.need_resched, etc. mustn't get changed by this CPU before it > returns to user- or fsys-mode, hence we disable interrupts early on." > [This may be a stale comment. I.e., we now check for resched needed in > current task's thread_info flags.] But, then I see later that when > CONFIG_PREEMPT is set, we do enable interrupts and then re-disable them. > > Then I notice that after we return from do_notify_resume_user() we don't > recheck TIF_NEED_RESCHED, etc. in the thread_info flags. So, maybe > this is the issue? If I open an interrupt window in > do_notify_resume_user(), 'NEED_RESCHED might get set and we wouldn't > notice it on return to the assembly language code. [I might even sleep > here. Is that a problem?] > > I took a look at the x86_64 and i386 versions and they always goes back > to check for resched after calling their version of do_notify_resume() > [no '_user suffix on the routine]. If this were the only issue with > enabling interrupts in do_notify_resume_user(), I could change it to > return non-zero if recheck is needed [zero in the usual case] and then > go back and recheck thread_info flags. > > Regards, > Lee > > > - > To unsubscribe from this list: send the line "unsubscribe linux-ia64" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- -Stephane