Re: [PATCH v6 13/13] riscv: Add qspinlock support

From: Guo Ren
Date: Fri Nov 29 2024 - 19:40:22 EST

On Fri, Nov 29, 2024 at 6:32 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> Hi everyone,
>
> On Fri, Nov 29, 2024 at 7:28 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
> >
> > Hi Conor & Alexandre,
> >
> > On Fri, Nov 29, 2024 at 10:58 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Nov 29, 2024 at 8:55 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
> > > >
> > > > On Fri, Nov 29, 2024 at 12:19 AM Conor Dooley <conor@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Thu, Nov 28, 2024 at 03:50:09PM +0100, Alexandre Ghiti wrote:
> > > > > > On 28/11/2024 15:14, Conor Dooley wrote:
> > > > > > > On Thu, Nov 28, 2024 at 01:41:36PM +0000, Will Deacon wrote:
> > > > > > > > On Thu, Nov 28, 2024 at 12:56:55PM +0000, Conor Dooley wrote:
> > > > > > > > > On Sun, Nov 03, 2024 at 03:51:53PM +0100, Alexandre Ghiti wrote:
> > > > > > > > > > In order to produce a generic kernel, a user can select
> > > > > > > > > > CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
> > > > > > > > > > spinlock implementation if Zabha or Ziccrse are not present.
> > > > > > > > > >
> > > > > > > > > > Note that we can't use alternatives here because the discovery of
> > > > > > > > > > extensions is done too late and we need to start with the qspinlock
> > > > > > > > > > implementation because the ticket spinlock implementation would pollute
> > > > > > > > > > the spinlock value, so let's use static keys.
> > > > > > > > > >
> > > > > > > > > > This is largely based on Guo's work and Leonardo reviews at [1].
> > > > > > > > > >
> > > > > > > > > > Link: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@xxxxxxxxxx/ [1]
> > > > > > > > > > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > > > > > > > > > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > > > > > > > > This patch (now commit ab83647fadae2 ("riscv: Add qspinlock support"))
> > > > > > > > > breaks boot on polarfire soc. It dies before outputting anything to the
> > > > > > > > > console. My .config has:
> > > > > > > > >
> > > > > > > > > # CONFIG_RISCV_TICKET_SPINLOCKS is not set
> > > > > > > > > # CONFIG_RISCV_QUEUED_SPINLOCKS is not set
> > > > > > > > > CONFIG_RISCV_COMBO_SPINLOCKS=y
> > > > > > > > I pointed out some of the fragility during review:
> > > > > > > >
> > > > > > > > https://lore.kernel.org/all/20241111164259.GA20042@willie-the-truck/
> > > > > > > >
> > > > > > > > so I'm kinda surprised it got merged tbh :/
> > > > > > > Maybe it could be reverted rather than having a broken boot with the
> > > > > > > default settings in -rc1.
> > > > > >
> > > > > >
> > > > > > No need to rush before we know what's happening,I guess you bisected to this
> > > > > > commit right?
> > > > >
> > > > > The symptom is a failure to boot, without any console output, of course
> > > > > I bisected it before blaming something specific. But I don't think it is
> > > > > "rushing" as having -rc1 broken with an option's default is a massive pain
> > > > > in the arse when it comes to testing.
> > > > >
> > > > > > I don't have this soc, so can you provide $stval/$sepc/$scause, a config, a
> > > > > > kernel, anything?
> > > > >
> > > > > I don't have the former cos it died immediately on boot. config is
> > > > > attached. It reproduces in QEMU so you don't need any hardware.
> > > > If QEMU could reproduce, could you provide a dmesg by the below method?
> > > >
> > > > Qemu cmd append: -s -S
> > > > ref: https://qemu-project.gitlab.io/qemu/system/gdb.html
> > > >
> > > > Connect gdb and in console:
> > > > 1. file vmlinux
> > > > 2. source ./Documentation/admin-guide/kdump/gdbmacros.txt
> > > > 3. dmesg
> > > >
> > > > Then, we could get the kernel's early boot logs from memory.
> > > I've reproduced it on qemu, thx for the config.
> > >
> > > Reading symbols from ../build-rv64lp64/vmlinux...
> > > (gdb) tar rem:1234
> > > Remote debugging using :1234
> > > ticket_spin_lock (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/include/asm-generic/ticket_spinlock.h:49
> > > 49 atomic_cond_read_acquire(&lock->val, ticket == (u16)VAL);
> > > (gdb) bt
> > > #0 ticket_spin_lock (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/include/asm-generic/ticket_spinlock.h:49
> > > #1 arch_spin_lock (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/arch/riscv/include/asm/spinlock.h:28
> > > #2 do_raw_spin_lock (lock=lock@entry=0xffffffff81b9a5b8 <text_mutex>)
> > > at /home/guoren/source/kernel/linux/kernel/locking/spinlock_debug.c:116
> > > #3 0xffffffff80b2ea0e in __raw_spin_lock_irqsave
> > > (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/include/linux/spinlock_api_smp.h:111
> > > #4 _raw_spin_lock_irqsave (lock=lock@entry=0xffffffff81b9a5b8
> > > <text_mutex>) at
> > > /home/guoren/source/kernel/linux/kernel/locking/spinlock.c:162
> > > #5 0xffffffff80b27c54 in rt_mutex_slowtrylock
> > > (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/kernel/locking/rtmutex.c:1393
> > > #6 0xffffffff80b295ea in rt_mutex_try_acquire
> > > (lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/kernel/locking/rtmutex.c:319
> > > #7 __rt_mutex_lock (state=2, lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/kernel/locking/rtmutex.c:1805
> > > #8 __mutex_lock_common (ip=18446744071562135170, nest_lock=0x0,
> > > subclass=0, state=2, lock=0xffffffff81b9a5b8 <text_mutex>) at
> > > /home/guoren/source/kernel/linux/kernel/locking/rtmutex_api.c:518
> > > #9 mutex_lock_nested (lock=0xffffffff81b9a5b8 <text_mutex>,
> > > subclass=subclass@entry=0) at
> > > /home/guoren/source/kernel/linux/kernel/locking/rtmutex_api.c:529
> > > #10 0xffffffff80010682 in arch_jump_label_transform_queue
> > > (entry=entry@entry=0xffffffff8158da28, type=<optimized out>) at
> > > /home/guoren/source/kernel/linux/arch/riscv/kernel/jump_label.c:39
> > > #11 0xffffffff801d86b2 in __jump_label_update
> > > (key=key@entry=0xffffffff81a1abb0 <qspinlock_key>,
> > > entry=0xffffffff8158da28, stop=stop@entry=0xffffffff815a5e68
> > > <__tracepoint_ptr_initcall_finish>, init=init@entry=true)
> > > at /home/guoren/source/kernel/linux/kernel/jump_label.c:513
> > > #12 0xffffffff801d890c in jump_label_update
> > > (key=key@entry=0xffffffff81a1abb0 <qspinlock_key>) at
> > > /home/guoren/source/kernel/linux/kernel/jump_label.c:920
> > > #13 0xffffffff801d8be8 in static_key_disable_cpuslocked
> > > (key=key@entry=0xffffffff81a1abb0 <qspinlock_key>) at
> > > /home/guoren/source/kernel/linux/kernel/jump_label.c:240
> > > #14 0xffffffff801d8c04 in static_key_disable
> > > (key=key@entry=0xffffffff81a1abb0 <qspinlock_key>) at
> > > /home/guoren/source/kernel/linux/kernel/jump_label.c:248
> > > #15 0xffffffff80c04a1a in riscv_spinlock_init () at
> > > /home/guoren/source/kernel/linux/arch/riscv/kernel/setup.c:271
> > > #16 setup_arch (cmdline_p=cmdline_p@entry=0xffffffff81a03e88) at
> > > /home/guoren/source/kernel/linux/arch/riscv/kernel/setup.c:336
> > > #17 0xffffffff80c007a2 in start_kernel () at
> > > /home/guoren/source/kernel/linux/init/main.c:922
> > > #18 0xffffffff80001164 in _start_kernel ()
> > > Backtrace stopped: frame did not save the PC
> > > (gdb) p /x lock
> > > $1 = 0xffffffff81b9a5b8
> > > (gdb) p /x *lock
> > > $2 = {{val = {counter = 0x20000}, {locked = 0x0, pending = 0x0},
> > > {locked_pending = 0x0, tail = 0x2}}}
> >
> > I have for you here a fast fixup for reference. (PS: I'm digging into
> > the root cause mentioned by Will Deacon.)
> >
> > diff --git a/arch/riscv/include/asm/text-patching.h
> > b/arch/riscv/include/asm/text-patching.h
> > index 7228e266b9a1..0439609f1cff 100644
> > --- a/arch/riscv/include/asm/text-patching.h
> > +++ b/arch/riscv/include/asm/text-patching.h
> > @@ -12,5 +12,6 @@ int patch_text_set_nosync(void *addr, u8 c, size_t len);
> > int patch_text(void *addr, u32 *insns, size_t len);
> >
> > extern int riscv_patch_in_stop_machine;
> > +extern int riscv_patch_in_spinlock_init;
> >
> > #endif /* _ASM_RISCV_PATCH_H */
> > diff --git a/arch/riscv/kernel/jump_label.c b/arch/riscv/kernel/jump_label.c
> > index 6eee6f736f68..d9a5a5c1933d 100644
> > --- a/arch/riscv/kernel/jump_label.c
> > +++ b/arch/riscv/kernel/jump_label.c
> > @@ -36,9 +36,11 @@ bool arch_jump_label_transform_queue(struct
> > jump_entry *entry,
> > insn = RISCV_INSN_NOP;
> > }
> >
> > - mutex_lock(&text_mutex);
> > + if (!riscv_patch_in_spinlock_init)
> > + mutex_lock(&text_mutex);
> > patch_insn_write(addr, &insn, sizeof(insn));
> > - mutex_unlock(&text_mutex);
> > + if (!riscv_patch_in_spinlock_init)
> > + mutex_unlock(&text_mutex);
> >
> > return true;
> > }
> > diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> > index db13c9ddf9e3..ab009cf855c2 100644
> > --- a/arch/riscv/kernel/patch.c
> > +++ b/arch/riscv/kernel/patch.c
> > @@ -24,6 +24,7 @@ struct patch_insn {
> > };
> >
> > int riscv_patch_in_stop_machine = false;
> > +int riscv_patch_in_spinlock_init = false;
> >
> > #ifdef CONFIG_MMU
> >
> > @@ -131,7 +132,7 @@ static int __patch_insn_write(void *addr, const
> > void *insn, size_t len)
> > * safe but triggers a lockdep failure, so just elide it for that
> > * specific case.
> > */
> > - if (!riscv_patch_in_stop_machine)
> > + if (!riscv_patch_in_stop_machine && !riscv_patch_in_spinlock_init)
> > lockdep_assert_held(&text_mutex);
> >
> > preempt_disable();
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index 016b48fcd6f2..87ddf1702be4 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -268,7 +268,9 @@ static void __init riscv_spinlock_init(void)
> > }
> > #if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> > else {
> > + riscv_patch_in_spinlock_init = 1;
> > static_branch_disable(&qspinlock_key);
> > + riscv_patch_in_spinlock_init = 0;
> > pr_info("Ticket spinlock: enabled\n");
> > return;
> > }
> >
> >
> >
> > --
> > Best Regards
> > Guo Ren
>
> Thanks Guo for looking into this.
>
> Your solution is not very pretty but I don't have anything better :/
Here is another solution (Only one file modified, maybe better):

diff --git a/arch/riscv/kernel/jump_label.c b/arch/riscv/kernel/jump_label.c
index 6eee6f736f68..654ed159c830 100644
--- a/arch/riscv/kernel/jump_label.c
+++ b/arch/riscv/kernel/jump_label.c
@@ -36,9 +36,15 @@ bool arch_jump_label_transform_queue(struct
jump_entry *entry,
insn = RISCV_INSN_NOP;
}

- mutex_lock(&text_mutex);
- patch_insn_write(addr, &insn, sizeof(insn));
- mutex_unlock(&text_mutex);
+ if (early_boot_irqs_disabled) {
+ riscv_patch_in_stop_machine = 1;
+ patch_insn_write(addr, &insn, sizeof(insn));
+ riscv_patch_in_stop_machine = 0;
+ } else {
+ mutex_lock(&text_mutex);
+ patch_insn_write(addr, &insn, sizeof(insn));
+ mutex_unlock(&text_mutex);
+ }

return true;
}

> Unless introducing a static_branch_XXX_nolock() API? I gave it a try
> and it fixes the issue, but not sure this will be accepted.
>
> The thing is the usage of static branches is temporary, we'll use
> alternatives when I finish working on getting the extensions very
The "alternatives" also need patch codes one by one, which means it
will meet the same problem as the jump_label.

So, you will still provide a patch like the one above for the
alternative implementation.

> early from the ACPI tables (I have a poc that works, just needs some
> cleaning).
>
> So let's say that I make this early extension parsing my priority for
> 6.14, can we live with Guo's hack in this release? Or should we revert
> this commit?
>
> Thanks,
>
> Alex
--
Best Regards
Guo Ren