Date: Tue, 19 Oct 2010 00:41:57 +0300 From: Alexander Motin <mav@FreeBSD.org> To: Marius Strobl <marius@alchemy.franken.de> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, John Baldwin <jhb@freebsd.org> Subject: Re: svn commit: r213985 - head/sys/sparc64/sparc64 Message-ID: <4CBCBF25.8010902@FreeBSD.org> In-Reply-To: <20101018213055.GP1416@alchemy.franken.de> References: <201010171646.o9HGks2U038501@svn.freebsd.org> <4CBCADDD.5070109@FreeBSD.org> <20101018205224.GO1416@alchemy.franken.de> <201010181705.24879.jhb@freebsd.org> <20101018213055.GP1416@alchemy.franken.de>
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Marius Strobl wrote: > On Mon, Oct 18, 2010 at 05:05:24PM -0400, John Baldwin wrote: >> On Monday, October 18, 2010 4:52:24 pm Marius Strobl wrote: >>> AFAICT this is not true; intr_event_handle() in sys/kern/kern_intr.c >>> is what enters a critical section and f.e. on amd64 I don't see where >>> anywhere in the path from ISR_VEC() to intr_execute_handlers() >>> calling intr_event_handle() a critical section would be entered, >>> which also means that in intr_execute_handlers() td_intr_nesting_level >>> is incremented outside of a critical section. >> Not all of the clock interrupts use intr_event_handle(). The local APIC >> timer uses its own interrupt entry point on x86 for example and uses an >> explicit critical section as a result. I suspect the sparc64 tick interrupt >> is closer to the local APIC timer case and doesn't use intr_event_handle(). > > Correct; but still you can't say that the MD interrupt code enters a > critical section in general, neither is incrementing td_intr_nesting_level > in intr_execute_handlers() protected by a critical section. > >> The fact that some clock interrupts do use intr_event_handle() (e.g. the >> atrtc driver on x86 now) does indicate that the low-level interrupt code >> probably does not belong in the time events code but in the caller. > > Well, I agree that entering a critical section in the time events > code would mean entering a nested critical section unnecessarily in > case the clock driver uses a regular "fast" interrupt handler and > that should be avoided. Still I don't think the event time front-end > actually should need to worry about wrapping the callback in a > critical section. Interrupt frame, required for hard-/stat-/profclock() operation is stored in curthread. So critical section is effectively mandatory there now. Correct td_intr_nesting_level value is also important for proper interrupt threads scheduling - one more reason to have critical section there. It is indeed strange that td_intr_nesting_level in intr_event_handle() is not covered by critical section, but probably it should. -- Alexander Motin
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