[PATCH 1/2] lib: sbi_init: Avoid thundering hurd problem with coldbook_lock
Anup Patel
anup at brainfault.org
Sun Aug 16 12:59:45 EDT 2020
On Sun, Aug 16, 2020 at 9:30 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>
> On 16 Aug 2020, at 16:45, Anup Patel <anup at brainfault.org> wrote:
> >
> > On Sun, Aug 16, 2020 at 8:18 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>
> >> On 16 Aug 2020, at 15:24, Anup Patel <anup at brainfault.org> wrote:
> >>>
> >>> On Sun, Aug 16, 2020 at 7:43 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>
> >>>> On 16 Aug 2020, at 15:02, Anup Patel <anup at brainfault.org> wrote:
> >>>>>
> >>>>> On Sun, Aug 16, 2020 at 12:20 PM Bin Meng <bmeng.cn at gmail.com> wrote:
> >>>>>>
> >>>>>> On Sun, Aug 16, 2020 at 3:49 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>>>>
> >>>>>>> On 15 Aug 2020, at 18:00, Anup Patel <anup.patel at wdc.com> wrote:
> >>>>>>>>
> >>>>>>>> We can have thundering hurd problem with coldboot_lock where the
> >>>>>>>> boot HART can potentially starve trying to acquire coldboot_lock
> >>>>>>>> because some of the non-boot HARTs are continuously acquiring and
> >>>>>>>> releasing coldboot_lock. This can happen if WFI is a NOP
> >>>>>>>
> >>>>>>> That is neither necessary nor sufficient, it's solely based on
> >>>>>>> whether the hart believes an M-mode software interrupt is pending?
> >>>>>>>
> >>>>>>>> OR if
> >>>>>>>> MIP.MSIP bit is already set for some of the non-boot HARTs.
> >>>>>>>>
> >>>>>>>> To avoid thundering hurd problem for coldboot_lock, we convert
> >>>>>>>> coldboot_done flag into atomic variable and using coldboot_lock
> >>>>>>>> only for coldboot_wait_hmask.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Anup Patel <anup.patel at wdc.com>
> >>>>>>>> Tested-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
> >>>>>>>> ---
> >>>>>>>> lib/sbi/sbi_init.c | 19 ++++++++++++-------
> >>>>>>>> 1 file changed, 12 insertions(+), 7 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
> >>>>>>>> index a7fb848..6b58983 100644
> >>>>>>>> --- a/lib/sbi/sbi_init.c
> >>>>>>>> +++ b/lib/sbi/sbi_init.c
> >>>>>>>> @@ -85,9 +85,10 @@ static void sbi_boot_prints(struct sbi_scratch *scratch, u32 hartid)
> >>>>>>>> }
> >>>>>>>>
> >>>>>>>> static spinlock_t coldboot_lock = SPIN_LOCK_INITIALIZER;
> >>>>>>>> -static unsigned long coldboot_done = 0;
> >>>>>>>> static struct sbi_hartmask coldboot_wait_hmask = { 0 };
> >>>>>>>>
> >>>>>>>> +static atomic_t coldboot_done = ATOMIC_INITIALIZER(0);
> >>>>>>>> +
> >>>>>>>> static void wait_for_coldboot(struct sbi_scratch *scratch, u32 hartid)
> >>>>>>>> {
> >>>>>>>> unsigned long saved_mie, cmip;
> >>>>>>>> @@ -105,16 +106,20 @@ static void wait_for_coldboot(struct sbi_scratch *scratch, u32 hartid)
> >>>>>>>> /* Mark current HART as waiting */
> >>>>>>>> sbi_hartmask_set_hart(hartid, &coldboot_wait_hmask);
> >>>>>>>>
> >>>>>>>> + /* Release coldboot lock */
> >>>>>>>> + spin_unlock(&coldboot_lock);
> >>>>>>>> +
> >>>>>>>> /* Wait for coldboot to finish using WFI */
> >>>>>>>> - while (!coldboot_done) {
> >>>>>>>> - spin_unlock(&coldboot_lock);
> >>>>>>>> + while (!atomic_read(&coldboot_done)) {
> >>>>>>>
> >>>>>>> Personally I'd make this a relaxed read and then explicitly fence
> >>>>>>> outside the loop, as otherwise if we end up with MSIP erroneously set
> >>>>>>> there may be a lot of cache coherency traffic due to repeated
> >>>>>>> unnecessary fences?
> >>>>>>>
> >>>>>>>> do {
> >>>>>>>> wfi();
> >>>>>>>> cmip = csr_read(CSR_MIP);
> >>>>>>>> } while (!(cmip & MIP_MSIP));
> >>>>>>>> - spin_lock(&coldboot_lock);
> >>>>>>>> };
> >>>>>>>>
> >>>>>>>> + /* Acquire coldboot lock */
> >>>>>>>> + spin_lock(&coldboot_lock);
> >>>>>>>> +
> >>>>>>>> /* Unmark current HART as waiting */
> >>>>>>>> sbi_hartmask_clear_hart(hartid, &coldboot_wait_hmask);
> >>>>>>>>
> >>>>>>>> @@ -132,12 +137,12 @@ static void wake_coldboot_harts(struct sbi_scratch *scratch, u32 hartid)
> >>>>>>>> {
> >>>>>>>> const struct sbi_platform *plat = sbi_platform_ptr(scratch);
> >>>>>>>>
> >>>>>>>> + /* Mark coldboot done */
> >>>>>>>> + atomic_write(&coldboot_done, 1);
> >>>>>>>
> >>>>>>> This only needs to be a store release.
> >>>>>>
> >>>>>> I believe relaxed read / write can work as well.
> >>>>>
> >>>>> Even if we use relaxed read / write, we will still need explicit Acquire
> >>>>> and Release barriers. Better to use __smp_store_release() and
> >>>>> __smp_load_acquire().
> >>>>
> >>>> Why not just use the C11 versions? Inline assembly is rarely needed
> >>>> these days for atomics. Especially helpful given the large hammer that
> >>>> is volatile inline assembly with a memory clobber causes highly
> >>>> pessimistic code generation from the compiler. In general it should
> >>>> only be necessary when needing to deal with I/O memory.
> >>>
> >>> We want to keep our barrier usage close to Linux RISC-V so that
> >>> people familiar with Linux can easily debug. Nothing against C11.
> >>
> >> Ok, but what about those who don't deal with Linux and have to go look
> >> up the semantics? C11/C++11 atomics are completely target-independent
> >> and a universal standard. There are far more developers out there who
> >> know C11/C++11 atomics and how to use them properly than those who know
> >> the Linux atomics. They are also, in my opinion, far easier to reason
> >> about, and the Linux ones are just poorly named (e.g. wmb() being
> >> `fence ow, ow` but smp_wmb() being `fence w, w` is, to my mind,
> >> confusing and not well-indicated by the names); I think it is far
> >> easier for a Linux developer to move to the C11/C++11 atomics world
> >> than the other way round. And, as I've said, using `__asm__
> >> __volatile__ ("fence ..." ::: "memory")` everywhere does seriously hurt
> >> the compiler's ability to optimise your code, whereas if you use
> >> C11/C++11 atomics then it knows exactly what you're doing.
> >
> > Like I said, nothing against C11 atomics. Moving to C11 atomics in
> > OpenSBI is a separate topic so feel free to send PATCHs for it.
>
> That is not the sentiment that most would extract from "We want to keep
> our barrier usage close to Linux RISC-V so that people familiar with
> Linux can easily debug". Such a statement comes across as saying that
> sending in C11 patches would be a waste of time because OpenSBI has
> already decided on staying with Linux atomics. Saying "nothing against
> C11 atomics" is also a bit of a vacuous statement as it omits the key
> "but", namely "but they are not Linux atomics which we prefer", so
> somewhat mis-represents your position, or at least the position you
> initially stated.
>
> >> Quite frankly, settling on Linux atomics in order to make it familiar
> >> to Linux developers is a bit hostile towards non-Linux developers.
> >> FreeBSD doesn't use Linux atomics (and in fact it uses its own set that
> >> are basically C11/C++11 atomics, just currently implemented in assembly
> >> because they date from before widespread C11 support in compilers, but
> >> they can all be replaced with their C11 equivalents, with the exception
> >> of things dealing with I/O), so you're effectively declaring that Linux
> >> matters more than any other operating system? Whether or not that's
> >> your intent I don't know, nor do I particularly care, but it's
> >> nonetheless how it comes across. Picking a universal standard ensures
> >> you don't express favouritism, whilst making it _more_ accessible
> >> overall.
> >
> > Most of the OpenSBI contributors have a back-ground of Linux kernel
> > development hence OpenSBI sources have a lot of Linux influence. This
> > does not mean we are against non-Linux ways of doing things.
>
> See above. I can completely understand starting out with Linux atomics
> if that's what the original developers are most familiar with, but that
> is a rather different position than what you first presented.
>
> > Your implication that "Linux matters more to OpenSBI" is unwarranted.
>
> Please do not mince my words, I chose them very carefully. I did not
> say "Linux matters more to OpenSBI", I said:
>
> you're effectively declaring that Linux
> matters more than any other operating system? Whether or not that's
> your intent I don't know, nor do I particularly care, but it's
> nonetheless how it comes across.
You are making an incorrect conclusion here. I am not commenting
any more on this.
>
> Note that second sentence; I explicitly stated that I was not accusing
> you of stating that Linux matters more, rather that your statements
> have the effect of _coming across_ that way _regardless_ of intent.
>
> >> Using __smp_load_acquire/__smp_store_release though does seem
> >> especially pointless; those are just obfuscated (to the compiler)
> >> versions of C11 atomics, so those at least should just be the C11
> >> versions, even if you do end up keeping the barriers.
> >
> > Like I mentioned, moving to C11 atomics is a separate topic
>
> Yes and no. The original patch used C11 atomics and when I suggested
> using acquire/release you then changed to not using C11 atomics. Using
> C11 atomics more widely, sure, that can be a separate thread, but given
> C11 atomics have already been introduced by yourself in this thread I
> think it's appropriate to discuss their use for this specific patch.
I am not familiar with C11 atomics. The riscv_atomic.c and riscv_atomic.h
have atomic operations inspired from Linux sources.
The coldboot_done usage is classing single-producer and multiple-consumer
problem so making coldboot_done as atomic seems overkill. That's why
moving to __smp_load_acquire()/__smp_store_release() is appropriate
for coldboot_done.
Regards,
Anup
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