linux-kernel - Re: [PATCH] srcu: Use suitable gfp_flags for the init_srcu_struct

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Message-ID: <37b5095a7c5014a5bcc9874b8433a043c8b94819@linux.dev>
Date: Sun, 14 Dec 2025 08:09:27 +0000
From: "Zqiang" <qiang.zhang@...ux.dev>
To: "Joel Fernandes" <joelagnelf@...dia.com>
Cc: paulmck@...nel.org, frederic@...nel.org, neeraj.upadhyay@...nel.org,
 rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] srcu: Use suitable gfp_flags for the
 init_srcu_struct_nodes()

> 
> > 
> > On Dec 14, 2025, at 12:28 PM, Zqiang <qiang.zhang@...ux.dev> wrote:
> >  
> >  
> > 
> > > 
> > > 
> > > 
> >  
> >  On Dec 13, 2025, at 10:56 PM, Zqiang <qiang.zhang@...ux.dev> wrote:
> >  
> >  In some kernels which is set convert_to_big to SRCU_SIZING_INIT,
> >  for use the init_srcu_struct*() to initialized srcu structure,
> >  the is_static parameters is always false, the memory allocation
> >  for srcu_sup structure's->node can use GFP_KERNEL flags.
> >  
> >  Signed-off-by: Zqiang <qiang.zhang@...ux.dev>
> >  ---
> >  kernel/rcu/srcutree.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >  
> >  diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> >  index ea3f128de06f..e4571b569752 100644
> >  --- a/kernel/rcu/srcutree.c
> >  +++ b/kernel/rcu/srcutree.c
> >  @@ -262,7 +262,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> >  ssp->srcu_sup->srcu_gp_seq_needed_exp = SRCU_GP_SEQ_INITIAL_VAL;
> >  ssp->srcu_sup->srcu_last_gp_end = ktime_get_mono_fast_ns();
> >  if (READ_ONCE(ssp->srcu_sup->srcu_size_state) == SRCU_SIZE_SMALL && SRCU_SIZING_IS_INIT()) {
> >  - if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC))
> >  + if (!init_srcu_struct_nodes(ssp, !is_static ? GFP_KERNEL : GFP_ATOMIC))
> >  
> > 
> > > 
> > > Nit: please avoid double negatives, becomes a bit harder to read:
> > >  
> > >  Instead,
> > >  is_static ? GFP_ATOMIC : GFP_KERNEL
> > > 
> >  
> >  Ok, will do that.
> >  
> > 
> > > 
> > > Is it also worthwhile adding a might_sleep() here for additional robustness?
> > > 
> >  
> >  Would it be more appropriate to add might_sleep() before
> >  allocating ssp->srcu_sup ?
> > 
> Actually this is probably not needed because the slab allocator already does that for sleepable allocations.

Yes, I also find the might_alloc() already exists in the slub allocator. 

Thanks
Zqiang

> 
> So feel free to ignore the suggestion.;-)
> 
> The one reason to do it might just be for documentation.
> 
> Thanks.
>