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Message-Id: <55642d365fc4272c4f43a88ea435e360@biophys.uni-duesseldorf.de>
Date: Tue, 12 Nov 2013 19:56:19 +1300
From: Michael Schmitz <schmitz@...phys.uni-duesseldorf.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Linux/m68k <linux-m68k@...r.kernel.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [patch 1/6] hardirq: Make hardirq bits generic
Thomas,
>> Finally found the issue. The patch below fixes the problem here. The
>> little missing detail is, that I zapped GET_CURRENT() assuming blindly
>> that this is only needed for the preempt_count hackery. But in fact
>> the world and some more depends on it which leads to interesting
>> explosions.
Thanks for debugging this - I won't speculate on why we attempt to
handle softirqs explicitly, as I have only a passing acquaintance with
this code, many years back.
>
> Some more thoughts on this.
>
> The whole nesting check in the exisiting low level entry code and what
> I tried to resemble with the irq_exit_nested() is pretty pointless.
>
> Let's look at auto_inthandler and ret_from_exception
>
> ENTRY(auto_inthandler)
> SAVE_ALL_INT
> GET_CURRENT(%d0)
> movel %d0,%a1
> addqb #1,%a1@(TINFO_PREEMPT+1)
> | put exception # in d0
> bfextu %sp@(PT_OFF_FORMATVEC){#4,#10},%d0
> subw #VEC_SPUR,%d0
>
> movel %sp,%sp@-
> movel %d0,%sp@- | put vector # on stack
> auto_irqhandler_fixup = . + 2
> jsr do_IRQ | process the IRQ
> addql #8,%sp | pop parameters off stack
>
> ret_from_interrupt:
> movel %curptr@(TASK_STACK),%a1
> subqb #1,%a1@(TINFO_PREEMPT+1)
> jeq ret_from_last_interrupt
> 2: RESTORE_ALL
>
> ALIGN
> ret_from_last_interrupt:
> moveq #(~ALLOWINT>>8)&0xff,%d0
> andb %sp@(PT_OFF_SR),%d0
> jne 2b
>
> /* check if we need to do software interrupts */
> tstl irq_stat+CPUSTAT_SOFTIRQ_PENDING
> jeq .Lret_from_exception
> pea ret_from_exception
> jra do_softirq
>
>
> ENTRY(ret_from_exception)
> .Lret_from_exception:
> btst #5,%sp@(PT_OFF_SR) | check if returning to kernel
> bnes 1f | if so, skip resched, signals
> ....
> 1: RESTORE_ALL
>
> So in every interrupt exit path we check:
>
> 1) Whether the hardirq part of preempt_count is zero
>
> 2) Whether the interrupt prio mask of SR on stack is zero
>
> and if we finally reach ret_from_exception we have the final check:
>
> 3) whether we return to kernel or user space.
>
> And this final check is the only one which matters, really.
>
> If you look at the probability of the first two checks catching
> anything, then it's pretty low. Most interrupts returns go through
> ret_from_exception. Yes, I added counters which prove that at least on
> the aranym, but I doubt that it will make a real difference if you run
> this on real hardware.
I'm happy to try that (indeed, verify your patch works on Atari
hardware first) - I trust the patch is relative to m68k git, not your
previous patches?
Cheers,
Michael
>
> So what's the point of having these checks in the hotpath? The patch
> below against 3.12 vanilla works nicely on the aranym and I don't see
> a reason why this extra hackery is necessary at all. It's just code
> bloat in a hotpath.
>
> Now the only valid concern might be do_softirq itself, but that's
> pointless as well. If the softirq is interrupted, then we do not
> invoke it again. If the nested interrupt happens before irq_exit()
> actually disables interrupts, then we won't invoke it either as the
> hardirq part of preempt_count is still not zero.
>
> As a side note: The do_softirq calls in the platform/68xxx entry
> pathes are just copied leftovers as well. Both entry code pathes are
> not fiddling with the preempt count and both call do_IRQ() which will
> call irq_exit() at the end which will invoke do_softirq(), so the
> check for more softirqs in the irq return path is just pointless.
>
> Thanks,
>
> tglx
> ---
> kernel/entry.S | 40 ++++------------------------------------
> kernel/ints.c | 6 ------
> platform/68000/entry.S | 33 ++++++++-------------------------
> platform/68360/entry.S | 24 +++---------------------
> 4 files changed, 15 insertions(+), 88 deletions(-)
>
>
> Index: linux-2.6/arch/m68k/kernel/entry.S
> ===================================================================
> --- linux-2.6.orig/arch/m68k/kernel/entry.S
> +++ linux-2.6/arch/m68k/kernel/entry.S
> @@ -45,7 +45,7 @@
> .globl system_call, buserr, trap, resume
> .globl sys_call_table
> .globl __sys_fork, __sys_clone, __sys_vfork
> -.globl ret_from_interrupt, bad_interrupt
> +.globl bad_interrupt
> .globl auto_irqhandler_fixup
> .globl user_irqvec_fixup
>
> @@ -275,8 +275,6 @@ do_delayed_trace:
> ENTRY(auto_inthandler)
> SAVE_ALL_INT
> GET_CURRENT(%d0)
> - movel %d0,%a1
> - addqb #1,%a1@(TINFO_PREEMPT+1)
> | put exception # in d0
> bfextu %sp@(PT_OFF_FORMATVEC){#4,#10},%d0
> subw #VEC_SPUR,%d0
> @@ -286,32 +284,13 @@ ENTRY(auto_inthandler)
> auto_irqhandler_fixup = . + 2
> jsr do_IRQ | process the IRQ
> addql #8,%sp | pop parameters off stack
> -
> -ret_from_interrupt:
> - movel %curptr@(TASK_STACK),%a1
> - subqb #1,%a1@(TINFO_PREEMPT+1)
> - jeq ret_from_last_interrupt
> -2: RESTORE_ALL
> -
> - ALIGN
> -ret_from_last_interrupt:
> - moveq #(~ALLOWINT>>8)&0xff,%d0
> - andb %sp@(PT_OFF_SR),%d0
> - jne 2b
> -
> - /* check if we need to do software interrupts */
> - tstl irq_stat+CPUSTAT_SOFTIRQ_PENDING
> - jeq .Lret_from_exception
> - pea ret_from_exception
> - jra do_softirq
> + jra ret_from_exception
>
> /* Handler for user defined interrupt vectors */
>
> ENTRY(user_inthandler)
> SAVE_ALL_INT
> GET_CURRENT(%d0)
> - movel %d0,%a1
> - addqb #1,%a1@(TINFO_PREEMPT+1)
> | put exception # in d0
> bfextu %sp@(PT_OFF_FORMATVEC){#4,#10},%d0
> user_irqvec_fixup = . + 2
> @@ -321,29 +300,18 @@ user_irqvec_fixup = . + 2
> movel %d0,%sp@- | put vector # on stack
> jsr do_IRQ | process the IRQ
> addql #8,%sp | pop parameters off stack
> -
> - movel %curptr@(TASK_STACK),%a1
> - subqb #1,%a1@(TINFO_PREEMPT+1)
> - jeq ret_from_last_interrupt
> - RESTORE_ALL
> + jra ret_from_exception
>
> /* Handler for uninitialized and spurious interrupts */
>
> ENTRY(bad_inthandler)
> SAVE_ALL_INT
> GET_CURRENT(%d0)
> - movel %d0,%a1
> - addqb #1,%a1@(TINFO_PREEMPT+1)
>
> movel %sp,%sp@-
> jsr handle_badint
> addql #4,%sp
> -
> - movel %curptr@(TASK_STACK),%a1
> - subqb #1,%a1@(TINFO_PREEMPT+1)
> - jeq ret_from_last_interrupt
> - RESTORE_ALL
> -
> + jra ret_from_exception
>
> resume:
> /*
> Index: linux-2.6/arch/m68k/kernel/ints.c
> ===================================================================
> --- linux-2.6.orig/arch/m68k/kernel/ints.c
> +++ linux-2.6/arch/m68k/kernel/ints.c
> @@ -58,12 +58,6 @@ void __init init_IRQ(void)
> {
> int i;
>
> - /* assembly irq entry code relies on this... */
> - if (HARDIRQ_MASK != 0x00ff0000) {
> - extern void hardirq_mask_is_broken(void);
> - hardirq_mask_is_broken();
> - }
> -
> for (i = IRQ_AUTO_1; i <= IRQ_AUTO_7; i++)
> irq_set_chip_and_handler(i, &auto_irq_chip, handle_simple_irq);
>
> Index: linux-2.6/arch/m68k/platform/68000/entry.S
> ===================================================================
> --- linux-2.6.orig/arch/m68k/platform/68000/entry.S
> +++ linux-2.6/arch/m68k/platform/68000/entry.S
> @@ -27,7 +27,6 @@
> .globl ret_from_exception
> .globl ret_from_signal
> .globl sys_call_table
> -.globl ret_from_interrupt
> .globl bad_interrupt
> .globl inthandler1
> .globl inthandler2
> @@ -137,7 +136,7 @@ inthandler1:
> movel #65,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler2:
> SAVE_ALL_INT
> @@ -148,7 +147,7 @@ inthandler2:
> movel #66,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler3:
> SAVE_ALL_INT
> @@ -159,7 +158,7 @@ inthandler3:
> movel #67,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler4:
> SAVE_ALL_INT
> @@ -170,7 +169,7 @@ inthandler4:
> movel #68,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler5:
> SAVE_ALL_INT
> @@ -181,7 +180,7 @@ inthandler5:
> movel #69,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler6:
> SAVE_ALL_INT
> @@ -192,7 +191,7 @@ inthandler6:
> movel #70,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler7:
> SAVE_ALL_INT
> @@ -203,7 +202,7 @@ inthandler7:
> movel #71,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> + bra ret_from_exception
>
> inthandler:
> SAVE_ALL_INT
> @@ -214,23 +213,7 @@ inthandler:
> movel %d0,%sp@- /* put vector # on stack*/
> jbsr process_int /* process the IRQ*/
> 3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> -
> -ret_from_interrupt:
> - jeq 1f
> -2:
> - RESTORE_ALL
> -1:
> - moveb %sp@(PT_OFF_SR), %d0
> - and #7, %d0
> - jhi 2b
> -
> - /* check if we need to do software interrupts */
> - jeq ret_from_exception
> -
> - pea ret_from_exception
> - jra do_softirq
> -
> + bra ret_from_exception
>
> /*
> * Handler for uninitialized and spurious interrupts.
> Index: linux-2.6/arch/m68k/platform/68360/entry.S
> ===================================================================
> --- linux-2.6.orig/arch/m68k/platform/68360/entry.S
> +++ linux-2.6/arch/m68k/platform/68360/entry.S
> @@ -29,7 +29,6 @@
> .globl ret_from_exception
> .globl ret_from_signal
> .globl sys_call_table
> -.globl ret_from_interrupt
> .globl bad_interrupt
> .globl inthandler
>
> @@ -132,26 +131,9 @@ inthandler:
>
> movel %sp,%sp@-
> movel %d0,%sp@- /* put vector # on stack*/
> - jbsr do_IRQ /* process the IRQ*/
> -3: addql #8,%sp /* pop parameters off stack*/
> - bra ret_from_interrupt
> -
> -ret_from_interrupt:
> - jeq 1f
> -2:
> - RESTORE_ALL
> -1:
> - moveb %sp@(PT_OFF_SR), %d0
> - and #7, %d0
> - jhi 2b
> - /* check if we need to do software interrupts */
> -
> - movel irq_stat+CPUSTAT_SOFTIRQ_PENDING,%d0
> - jeq ret_from_exception
> -
> - pea ret_from_exception
> - jra do_softirq
> -
> + jbsr do_IRQ /* process the IRQ */
> + addql #8,%sp /* pop parameters off stack*/
> + jra ret_from_exception
>
> /*
> * Handler for uninitialized and spurious interrupts.
>
>
>
>
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