Re: [PATCH 1/1] sparc64: unify thread stack sizing and add explicit 32KB stack

[David Laight <david.laight.linux@gmail.com>]

On Tue, 16 Jun 2026 16:18:33 +0200
Andreas Larsson <andreas.larsson@gaisler.com> wrote:

> On 2026-05-19 09:57, Tony Rodriguez wrote:
> > This patch restructures the thread‑stack sizing logic into a single
> > if / elif / else chain and introduces an explicit 32KB kernel stack
> > for SPARC64. The previous implementation relied on nested conditionals
> > and PAGE_SHIFT‑dependent behavior, which produced 8KB or 16KB stacks
> > depending on configuration. SPARC64 requires a larger,
> > architecture‑specific stack due to its trapframe size, register‑window
> > behavior, and deeper call paths.
> > 
> > A reproducible failure case occurs when usbcore is enabled: USB hub
> > enumeration (usb_new_device(), hub_port_connect(), PM/QoS helpers)
> > allocates large on‑stack structures and recurses through several
> > layers of device‑model code. Combined with SPARC64’s trapframe and
> > register‑window overhead, this reliably exhausts a 16KB stack and
> > results in early‑boot panics.  A 32KB stack eliminates these failures.
> > 
> > The new logic is:
> >     SPARC64:
> >         THREAD_SIZE = 4 * PAGE_SIZE (32KB)
> >         THREAD_SHIFT = PAGE_SHIFT + 2 (log₂(32KB))
> >         THREAD_SIZE_ORDER = 2 (4 contiguous pages)  
> 
> Yes
> 
> >     Non‑SPARC64 with PAGE_SHIFT == 13:
> >         Retains the existing 16KB stack behavior
> >     Fallback:
> >         Retains the existing 8KB stack behavior  
> 
> No, not to my understanding, see comments below.
> 
> > 
> > Signed-off-by: Tony Rodriguez <unixpro1970@gmail.com>
> > ---
> >  arch/sparc/include/asm/thread_info_64.h | 28 ++++++++++++-------------
> >  1 file changed, 14 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/sparc/include/asm/thread_info_64.h b/arch/sparc/include/asm/thread_info_64.h
> > index c8a73dff27f8..6b12a2b66385 100644
> > --- a/arch/sparc/include/asm/thread_info_64.h
> > +++ b/arch/sparc/include/asm/thread_info_64.h
> > @@ -99,13 +99,20 @@ struct thread_info {
> >  #define FAULT_CODE_BLKCOMMIT	0x10	/* Use blk-commit ASI in copy_page */
> >  #define	FAULT_CODE_BAD_RA	0x20	/* Bad RA for sun4v		   */
> > 
> > -#if PAGE_SHIFT == 13
> > -#define THREAD_SIZE (2*PAGE_SIZE)
> > -#define THREAD_SHIFT (PAGE_SHIFT + 1)
> > -#else /* PAGE_SHIFT == 13 */
> > -#define THREAD_SIZE PAGE_SIZE
> > -#define THREAD_SHIFT PAGE_SHIFT
> > -#endif /* PAGE_SHIFT == 13 */
> > +/* thread information allocation */
> > +#ifdef CONFIG_SPARC64
> > +	#define THREAD_SIZE (4 * PAGE_SIZE)
> > +	#define THREAD_SHIFT (PAGE_SHIFT + 2)
> > +	#define THREAD_SIZE_ORDER 2  
> 
> As far as I can see, given that this header is included by
> 
> #if defined(__sparc__) && defined(__arch64__)
> #include <asm/thread_info_64.h>
> #else
> #include <asm/thread_info_32.h>
> #endif
> 
> the code above is the only code that will ever be compiled, while leaving...
> 
> > +#elif PAGE_SHIFT == 13
> > +	#define THREAD_SIZE (2 * PAGE_SIZE)
> > +	#define THREAD_SHIFT (PAGE_SHIFT + 1)
> > +	#define THREAD_SIZE_ORDER 1
> > +#else
> > +	#define THREAD_SIZE PAGE_SIZE
> > +	#define THREAD_SHIFT PAGE_SHIFT
> > +	#define THREAD_SIZE_ORDER 0
> > +#endif  
> 
> ...this code dead, where the else branch code already was dead (but then
> in two separate else braches).
> 
> I'd rather see the else branch here and the else branch below cleaned up
> by a separate patch with a fixup tag for commit 15b9350a177b ("sparc64:
> Only support 4MB huge pages and 8KB base pages.") that as far as I can
> see should have removed the else branch. The else branches was to use
> only one page when the page size was _larger_ than 8 KiB when that was
> an option.

That whole logic is impenetrable.
Why not set the 'desired thread size' in kB, then work out how many
pages that ends up being based on the page size, and finally get the actual
stack size.
I'm not sure, but with vmalloc()ed stacks and 8k pages can't you have 24kB?

	David


> 
> > 
> >  /*
> >   * macros/functions for gaining access to the thread information structure
> > @@ -127,13 +134,6 @@ register struct thread_info *current_thread_info_reg asm("g6");
> >  extern struct thread_info *current_thread_info(void);
> >  #endif
> > 
> > -/* thread information allocation */
> > -#if PAGE_SHIFT == 13
> > -#define THREAD_SIZE_ORDER	1
> > -#else /* PAGE_SHIFT == 13 */
> > -#define THREAD_SIZE_ORDER	0
> > -#endif /* PAGE_SHIFT == 13 */
> > -
> >  #define __thread_flag_byte_ptr(ti)	\
> >  	((unsigned char *)(&((ti)->flags)))
> >  #define __cur_thread_flag_byte_ptr	__thread_flag_byte_ptr(current_thread_info())
> > --
> > 2.53.0
> >   
> 
> Apart from the above I agree with David Laight that more investigation
> of the situation that leads to this problem would be good. Granted,
> sparc and sparc64 in particular is a bit special with its stack frames,
> but among other arches it seems to be uncommon with 32 KiB of thread
> stack unless KASAN is enabled.
> 
> Cheers,
> Andreas
>