2019-06-18 09:28:37

by Arnd Bergmann

Subject: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

gcc gets confused in pcpu_get_vm_areas() because there are too many
branches that affect whether 'lva' was initialized before it gets
used:

mm/vmalloc.c: In function 'pcpu_get_vm_areas':
mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
insert_vmap_area_augment(lva, &va->rb_node,
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
&free_vmap_area_root, &free_vmap_area_list);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/vmalloc.c:916:20: note: 'lva' was declared here
struct vmap_area *lva;
^~~

Add an intialization to NULL, and check whether this has changed
before the first use.

Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
Signed-off-by: Arnd Bergmann <[email protected]>
---
mm/vmalloc.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a9213fc3802d..42a6f795c3ee 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
unsigned long nva_start_addr, unsigned long size,
enum fit_type type)
{
- struct vmap_area *lva;
+ /*
+ * GCC cannot always keep track of whether this variable
+ * was initialized across many branches, therefore set
+ * it NULL here to avoid a warning.
+ */
+ struct vmap_area *lva = NULL;

if (type == FL_FIT_TYPE) {
/*
@@ -987,7 +992,7 @@ adjust_va_to_fit_type(struct vmap_area *va,
if (type != FL_FIT_TYPE) {
augment_tree_propagate_from(va);

- if (type == NE_FIT_TYPE)
+ if (lva)
insert_vmap_area_augment(lva, &va->rb_node,
&free_vmap_area_root, &free_vmap_area_list);
}
--
2.20.0

2019-06-18 13:42:51

by Joel Fernandes

Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <[email protected]> wrote:
>
> gcc gets confused in pcpu_get_vm_areas() because there are too many
> branches that affect whether 'lva' was initialized before it gets
> used:
>
> mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> insert_vmap_area_augment(lva, &va->rb_node,
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> &free_vmap_area_root, &free_vmap_area_list);
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/vmalloc.c:916:20: note: 'lva' was declared here
> struct vmap_area *lva;
> ^~~
>
> Add an intialization to NULL, and check whether this has changed
> before the first use.
>
> Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
> mm/vmalloc.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a9213fc3802d..42a6f795c3ee 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
> unsigned long nva_start_addr, unsigned long size,
> enum fit_type type)
> {
> - struct vmap_area *lva;
> + /*
> + * GCC cannot always keep track of whether this variable
> + * was initialized across many branches, therefore set
> + * it NULL here to avoid a warning.
> + */
> + struct vmap_area *lva = NULL;

Fair enough, but is this 5-line comment really needed here?

- Joel

2019-06-18 14:08:15

by Uladzislau Rezki (Sony)

Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, Jun 18, 2019 at 09:40:28AM -0400, Joel Fernandes wrote:
> On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <[email protected]> wrote:
> >
> > gcc gets confused in pcpu_get_vm_areas() because there are too many
> > branches that affect whether 'lva' was initialized before it gets
> > used:
> >
> > mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > insert_vmap_area_augment(lva, &va->rb_node,
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > &free_vmap_area_root, &free_vmap_area_list);
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > mm/vmalloc.c:916:20: note: 'lva' was declared here
> > struct vmap_area *lva;
> > ^~~
> >
> > Add an intialization to NULL, and check whether this has changed
> > before the first use.
> >
> > Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> > Signed-off-by: Arnd Bergmann <[email protected]>
> > ---
> > mm/vmalloc.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index a9213fc3802d..42a6f795c3ee 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
> > unsigned long nva_start_addr, unsigned long size,
> > enum fit_type type)
> > {
> > - struct vmap_area *lva;
> > + /*
> > + * GCC cannot always keep track of whether this variable
> > + * was initialized across many branches, therefore set
> > + * it NULL here to avoid a warning.
> > + */
> > + struct vmap_area *lva = NULL;
>
> Fair enough, but is this 5-line comment really needed here?
>
How it is rewritten now, probably not. I would just set it NULL and
leave the comment, but that is IMHO. Anyway

Reviewed-by: Uladzislau Rezki (Sony) <[email protected]>

Thanks!

--
Vlad Rezki

2019-06-18 20:59:49

by Andrew Morton

Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, 18 Jun 2019 16:06:22 +0200 Uladzislau Rezki <[email protected]> wrote:

> On Tue, Jun 18, 2019 at 09:40:28AM -0400, Joel Fernandes wrote:
> > On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <[email protected]> wrote:
> > >
> > > gcc gets confused in pcpu_get_vm_areas() because there are too many
> > > branches that affect whether 'lva' was initialized before it gets
> > > used:
> > >
> > > mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> > > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > insert_vmap_area_augment(lva, &va->rb_node,
> > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > &free_vmap_area_root, &free_vmap_area_list);
> > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > mm/vmalloc.c:916:20: note: 'lva' was declared here
> > > struct vmap_area *lva;
> > > ^~~
> > >
> > > Add an intialization to NULL, and check whether this has changed
> > > before the first use.
> > >
> > > Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> > > Signed-off-by: Arnd Bergmann <[email protected]>
> > > ---
> > > mm/vmalloc.c | 9 +++++++--
> > > 1 file changed, 7 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index a9213fc3802d..42a6f795c3ee 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
> > > unsigned long nva_start_addr, unsigned long size,
> > > enum fit_type type)
> > > {
> > > - struct vmap_area *lva;
> > > + /*
> > > + * GCC cannot always keep track of whether this variable
> > > + * was initialized across many branches, therefore set
> > > + * it NULL here to avoid a warning.
> > > + */
> > > + struct vmap_area *lva = NULL;
> >
> > Fair enough, but is this 5-line comment really needed here?
> >
> How it is rewritten now, probably not. I would just set it NULL and
> leave the comment, but that is IMHO. Anyway
>

I agree - given that the patch does this:

@@ -972,7 +977,7 @@ adjust_va_to_fit_type(struct vmap_area *
if (type != FL_FIT_TYPE) {
augment_tree_propagate_from(va);

- if (type == NE_FIT_TYPE)
+ if (lva)
insert_vmap_area_augment(lva, &va->rb_node,
&free_vmap_area_root, &free_vmap_area_list);
}

the comment simply isn't relevant any more. Although I guess this
might be a bit helpful:

@@ -977,7 +972,7 @@ adjust_va_to_fit_type(struct vmap_area *
if (type != FL_FIT_TYPE) {
augment_tree_propagate_from(va);

- if (lva)
+ if (lva) /* type == NE_FIT_TYPE */
insert_vmap_area_augment(lva, &va->rb_node,
&free_vmap_area_root, &free_vmap_area_list);
}

2019-06-19 10:37:27

by Uladzislau Rezki (Sony)

Subject: Re: [PATCH] mm/vmalloc: avoid bogus -Wmaybe-uninitialized warning

On Tue, Jun 18, 2019 at 01:59:20PM -0700, Andrew Morton wrote:
> On Tue, 18 Jun 2019 16:06:22 +0200 Uladzislau Rezki <[email protected]> wrote:
>
> > On Tue, Jun 18, 2019 at 09:40:28AM -0400, Joel Fernandes wrote:
> > > On Tue, Jun 18, 2019 at 5:27 AM Arnd Bergmann <[email protected]> wrote:
> > > >
> > > > gcc gets confused in pcpu_get_vm_areas() because there are too many
> > > > branches that affect whether 'lva' was initialized before it gets
> > > > used:
> > > >
> > > > mm/vmalloc.c: In function 'pcpu_get_vm_areas':
> > > > mm/vmalloc.c:991:4: error: 'lva' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > > insert_vmap_area_augment(lva, &va->rb_node,
> > > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > &free_vmap_area_root, &free_vmap_area_list);
> > > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > mm/vmalloc.c:916:20: note: 'lva' was declared here
> > > > struct vmap_area *lva;
> > > > ^~~
> > > >
> > > > Add an intialization to NULL, and check whether this has changed
> > > > before the first use.
> > > >
> > > > Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> > > > Signed-off-by: Arnd Bergmann <[email protected]>
> > > > ---
> > > > mm/vmalloc.c | 9 +++++++--
> > > > 1 file changed, 7 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index a9213fc3802d..42a6f795c3ee 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -913,7 +913,12 @@ adjust_va_to_fit_type(struct vmap_area *va,
> > > > unsigned long nva_start_addr, unsigned long size,
> > > > enum fit_type type)
> > > > {
> > > > - struct vmap_area *lva;
> > > > + /*
> > > > + * GCC cannot always keep track of whether this variable
> > > > + * was initialized across many branches, therefore set
> > > > + * it NULL here to avoid a warning.
> > > > + */
> > > > + struct vmap_area *lva = NULL;
> > >
> > > Fair enough, but is this 5-line comment really needed here?
> > >
> > How it is rewritten now, probably not. I would just set it NULL and
> > leave the comment, but that is IMHO. Anyway
> >
>
> I agree - given that the patch does this:
>
> @@ -972,7 +977,7 @@ adjust_va_to_fit_type(struct vmap_area *
> if (type != FL_FIT_TYPE) {
> augment_tree_propagate_from(va);
>
> - if (type == NE_FIT_TYPE)
> + if (lva)
> insert_vmap_area_augment(lva, &va->rb_node,
> &free_vmap_area_root, &free_vmap_area_list);
> }
>
> the comment simply isn't relevant any more. Although I guess this
> might be a bit helpful:
>
> @@ -977,7 +972,7 @@ adjust_va_to_fit_type(struct vmap_area *
> if (type != FL_FIT_TYPE) {
> augment_tree_propagate_from(va);
>
> - if (lva)
> + if (lva) /* type == NE_FIT_TYPE */
> insert_vmap_area_augment(lva, &va->rb_node,
> &free_vmap_area_root, &free_vmap_area_list);
> }
>
That comment makes it much clear, thanks!

--
Vlad Rezki