Re: [PATCHv3 perf/core 07/22] uprobes: Add do_ref_ctr argument to uprobe_write function

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Jun 25, 2025 at 03:42:59PM +0900, Masami Hiramatsu wrote:
> On Thu,  5 Jun 2025 15:23:34 +0200
> Jiri Olsa <jolsa@kernel.org> wrote:
> 
> > Making update_ref_ctr call in uprobe_write conditional based
> > on do_ref_ctr argument. This way we can use uprobe_write for
> > instruction update without doing ref_ctr_offset update.
> > 
> 
> Can we just decouple this update from uprobe_write()?
> If we do this exclusively, I think we can do something like;
> 
> lock()
> update_ref_ctr(uprobe, mm, +1);
> ...
> ret = uprobe_write();
> ...
> if (ret < 0)
>   update_ref_ctr(uprobe, mm, -1);
> unlock()
> 
> Thank you,

it was the intention in the v1 but as Oleg pointed out [1] it won't
work, because the set_orig_refctr part of this change does not know
if the refctr should be really updated or not

while inside uprobe_write it happens right after verify callback and
we make sure to update refctr only if the instruction was updated

thanks,
jirka


[1] https://lore.kernel.org/bpf/20250427141335.GA9350@redhat.com/

> 
> 
> > Acked-by: Oleg Nesterov <oleg@redhat.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  include/linux/uprobes.h | 2 +-
> >  kernel/events/uprobes.c | 8 ++++----
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > index 518b26756469..5080619560d4 100644
> > --- a/include/linux/uprobes.h
> > +++ b/include/linux/uprobes.h
> > @@ -200,7 +200,7 @@ extern unsigned long uprobe_get_trap_addr(struct pt_regs *regs);
> >  extern int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma, unsigned long vaddr, uprobe_opcode_t,
> >  			       bool is_register);
> >  extern int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma, const unsigned long opcode_vaddr,
> > -			uprobe_opcode_t *insn, int nbytes, uprobe_write_verify_t verify, bool is_register);
> > +			uprobe_opcode_t *insn, int nbytes, uprobe_write_verify_t verify, bool is_register, bool do_update_ref_ctr);
> >  extern struct uprobe *uprobe_register(struct inode *inode, loff_t offset, loff_t ref_ctr_offset, struct uprobe_consumer *uc);
> >  extern int uprobe_apply(struct uprobe *uprobe, struct uprobe_consumer *uc, bool);
> >  extern void uprobe_unregister_nosync(struct uprobe *uprobe, struct uprobe_consumer *uc);
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 1e5dc3b30707..6795b8d82b9c 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -492,12 +492,12 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> >  		bool is_register)
> >  {
> >  	return uprobe_write(auprobe, vma, opcode_vaddr, &opcode, UPROBE_SWBP_INSN_SIZE,
> > -			    verify_opcode, is_register);
> > +			    verify_opcode, is_register, true /* do_update_ref_ctr */);
> >  }
> >  
> >  int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> >  		 const unsigned long insn_vaddr, uprobe_opcode_t *insn, int nbytes,
> > -		 uprobe_write_verify_t verify, bool is_register)
> > +		 uprobe_write_verify_t verify, bool is_register, bool do_update_ref_ctr)
> >  {
> >  	const unsigned long vaddr = insn_vaddr & PAGE_MASK;
> >  	struct mm_struct *mm = vma->vm_mm;
> > @@ -538,7 +538,7 @@ int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> >  	}
> >  
> >  	/* We are going to replace instruction, update ref_ctr. */
> > -	if (!ref_ctr_updated && uprobe->ref_ctr_offset) {
> > +	if (do_update_ref_ctr && !ref_ctr_updated && uprobe->ref_ctr_offset) {
> >  		ret = update_ref_ctr(uprobe, mm, is_register ? 1 : -1);
> >  		if (ret) {
> >  			folio_put(folio);
> > @@ -590,7 +590,7 @@ int uprobe_write(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> >  
> >  out:
> >  	/* Revert back reference counter if instruction update failed. */
> > -	if (ret < 0 && ref_ctr_updated)
> > +	if (do_update_ref_ctr && ret < 0 && ref_ctr_updated)
> >  		update_ref_ctr(uprobe, mm, is_register ? -1 : 1);
> >  
> >  	/* try collapse pmd for compound page */
> > -- 
> > 2.49.0
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>