[PATCH 0/2] eliminate the uninitialized compilation warning

[PATCH 0/2] eliminate the uninitialized compilation warning

[Hongbo Li]

Eliminating the uninitialized compilation warning in bcachefs-tools.

Hongbo Li (2):
  bcachefs: eliminate the uninitialized compilation warning in
    bch2_reconstruct_snapshots
  bcachefs: eliminate the uninitialized compilation warning in
    __do_six_trylock

 fs/bcachefs/six.c      | 6 ++----
 fs/bcachefs/snapshot.c | 5 ++++-
 2 files changed, 6 insertions(+), 5 deletions(-)

-- 
2.34.1

[PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Hongbo Li]

[BUG]
When compiling the bcachefs-tools, the following compilation warning
is reported:
    libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
    libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      915 |  snapshot->v.tree = cpu_to_le32(tree_id);
    libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
      903 |  u32 tree_id;
       |      ^~~~~~~

[CAUSE]
This is a false alert, because @tree_id is changed in
bch2_snapshot_tree_create after it returns 0. And if this function
returns other value, @tree_id wouldn't be used. Thus there should
be nothing wrong in logical.

[FIX]
Although the report itself is a false alert, we can still make it more
explicit by:
  - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
  - initialize @tree_id to U32_MAX
  - add extra WARN_ON_ONCE to make sure @tree_id is updated

Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
---
 fs/bcachefs/snapshot.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
index 0b26dee17a5a..cad3408903b2 100644
--- a/fs/bcachefs/snapshot.c
+++ b/fs/bcachefs/snapshot.c
@@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
 static int bch2_snapshot_tree_create(struct btree_trans *trans,
 				u32 root_id, u32 subvol_id, u32 *tree_id)
 {
+	WARN_ON_ONCE(!tree_id);
 	struct bkey_i_snapshot_tree *n_tree =
 		__bch2_snapshot_tree_create(trans);
 
@@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
 	if (bch2_snapshot_equiv(c, id))
 		return 0;
 
-	u32 tree_id;
+	u32 tree_id = U32_MAX;
 	int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
 	if (ret)
 		return ret;
@@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
 	if (ret)
 		return ret;
 
+	/* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */
+	WARN_ON_ONCE(tree_id == U32_MAX);
 	bkey_snapshot_init(&snapshot->k_i);
 	snapshot->k.p		= POS(0, id);
 	snapshot->v.tree	= cpu_to_le32(tree_id);
-- 
2.34.1

[PATCH 2/2] bcachefs: eliminate the uninitialized compilation warning in __do_six_trylock

[Hongbo Li]

When compiling the bcachefs-tools, the following compilation warning
is reported:
    libbcachefs/six.c: In function ‘__do_six_trylock’:
    libbcachefs/six.c:90:12: warning: ‘old’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       90 |  if (!(old & SIX_LOCK_HELD_intent)) {
        |       ~~~~~^~~~~~~~~~~~~~~~~~~~~~~

This is also a false altert. Only when @type=SIX_LOCK_write and @try=false
are passed in __do_six_trylock, the second condition branch would enter
which does not initialize the @old variable. But six_set_owner will not
use @old if @type is not SIX_LOCK_intent. There should be nothing wrong
in logical too.

Although the report itself is a false alert, we can elimate the unitialize
compilation warning by assigning @old in front.

Fixes: 84a37cbf62e0 ("six locks: Wakeup now takes lock on behalf of waiter")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
---
 fs/bcachefs/six.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/bcachefs/six.c b/fs/bcachefs/six.c
index 3a494c5d1247..9f782e4e3ca9 100644
--- a/fs/bcachefs/six.c
+++ b/fs/bcachefs/six.c
@@ -118,11 +118,11 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
 			    struct task_struct *task, bool try)
 {
 	int ret;
-	u32 old;
+	u32 old = atomic_read(&lock->state);
 
 	EBUG_ON(type == SIX_LOCK_write && lock->owner != task);
 	EBUG_ON(type == SIX_LOCK_write &&
-		(try != !(atomic_read(&lock->state) & SIX_LOCK_HELD_write)));
+		(try != !(old & SIX_LOCK_HELD_write)));
 
 	/*
 	 * Percpu reader mode:
@@ -157,7 +157,6 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
 
 		smp_mb();
 
-		old = atomic_read(&lock->state);
 		ret = !(old & l[type].lock_fail);
 
 		this_cpu_sub(*lock->readers, !ret);
@@ -182,7 +181,6 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
 				ret = -1 - SIX_LOCK_read;
 		}
 	} else {
-		old = atomic_read(&lock->state);
 		do {
 			ret = !(old & l[type].lock_fail);
 			if (!ret || (type == SIX_LOCK_write && !try)) {
-- 
2.34.1

Re: [PATCH 0/2] eliminate the uninitialized compilation warning

[Hongbo Li]

These are two compilation warning in bcachefs-tools, the patches may be 
useful.

Thanks,
Hongbo.

On 2024/4/19 15:48, Hongbo Li wrote:
> Eliminating the uninitialized compilation warning in bcachefs-tools.
> 
> Hongbo Li (2):
>    bcachefs: eliminate the uninitialized compilation warning in
>      bch2_reconstruct_snapshots
>    bcachefs: eliminate the uninitialized compilation warning in
>      __do_six_trylock
> 
>   fs/bcachefs/six.c      | 6 ++----
>   fs/bcachefs/snapshot.c | 5 ++++-
>   2 files changed, 6 insertions(+), 5 deletions(-)
> 

Re: [PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Kent Overstreet]

On Fri, Apr 19, 2024 at 03:48:50PM +0800, Hongbo Li wrote:
> [BUG]
> When compiling the bcachefs-tools, the following compilation warning
> is reported:
>     libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
>     libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>       915 |  snapshot->v.tree = cpu_to_le32(tree_id);
>     libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
>       903 |  u32 tree_id;
>        |      ^~~~~~~
> 
> [CAUSE]
> This is a false alert, because @tree_id is changed in
> bch2_snapshot_tree_create after it returns 0. And if this function
> returns other value, @tree_id wouldn't be used. Thus there should
> be nothing wrong in logical.
> 
> [FIX]
> Although the report itself is a false alert, we can still make it more
> explicit by:
>   - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
>   - initialize @tree_id to U32_MAX
>   - add extra WARN_ON_ONCE to make sure @tree_id is updated
> 
> Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
> Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
> ---
>  fs/bcachefs/snapshot.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
> index 0b26dee17a5a..cad3408903b2 100644
> --- a/fs/bcachefs/snapshot.c
> +++ b/fs/bcachefs/snapshot.c
> @@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
>  static int bch2_snapshot_tree_create(struct btree_trans *trans,
>  				u32 root_id, u32 subvol_id, u32 *tree_id)
>  {
> +	WARN_ON_ONCE(!tree_id);

There's no point checking for a null pointer like that; the oops from a
null ptr deref gives us exactly the same information.

>  	struct bkey_i_snapshot_tree *n_tree =
>  		__bch2_snapshot_tree_create(trans);
>  
> @@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>  	if (bch2_snapshot_equiv(c, id))
>  		return 0;
>  
> -	u32 tree_id;
> +	u32 tree_id = U32_MAX;

Just initialize it to 0. 0 is an invalid tree ID, so it'll be caught by
snapshot_tree_invalid() if it's not set.

>  	int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
>  	if (ret)
>  		return ret;
> @@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>  	if (ret)
>  		return ret;
>  
> +	/* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */
> +	WARN_ON_ONCE(tree_id == U32_MAX);

Nix this as well.

>  	bkey_snapshot_init(&snapshot->k_i);
>  	snapshot->k.p		= POS(0, id);
>  	snapshot->v.tree	= cpu_to_le32(tree_id);
> -- 
> 2.34.1
> 

Re: [PATCH 2/2] bcachefs: eliminate the uninitialized compilation warning in __do_six_trylock

[Kent Overstreet]

On Fri, Apr 19, 2024 at 03:48:51PM +0800, Hongbo Li wrote:
> When compiling the bcachefs-tools, the following compilation warning
> is reported:
>     libbcachefs/six.c: In function ‘__do_six_trylock’:
>     libbcachefs/six.c:90:12: warning: ‘old’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>        90 |  if (!(old & SIX_LOCK_HELD_intent)) {
>         |       ~~~~~^~~~~~~~~~~~~~~~~~~~~~~
> 
> This is also a false altert. Only when @type=SIX_LOCK_write and @try=false
> are passed in __do_six_trylock, the second condition branch would enter
> which does not initialize the @old variable. But six_set_owner will not
> use @old if @type is not SIX_LOCK_intent. There should be nothing wrong
> in logical too.
> 
> Although the report itself is a false alert, we can elimate the unitialize
> compilation warning by assigning @old in front.
> 
> Fixes: 84a37cbf62e0 ("six locks: Wakeup now takes lock on behalf of waiter")
> Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
> ---
>  fs/bcachefs/six.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/bcachefs/six.c b/fs/bcachefs/six.c
> index 3a494c5d1247..9f782e4e3ca9 100644
> --- a/fs/bcachefs/six.c
> +++ b/fs/bcachefs/six.c
> @@ -118,11 +118,11 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
>  			    struct task_struct *task, bool try)
>  {
>  	int ret;
> -	u32 old;
> +	u32 old = atomic_read(&lock->state);
>  
>  	EBUG_ON(type == SIX_LOCK_write && lock->owner != task);
>  	EBUG_ON(type == SIX_LOCK_write &&
> -		(try != !(atomic_read(&lock->state) & SIX_LOCK_HELD_write)));
> +		(try != !(old & SIX_LOCK_HELD_write)));
>  
>  	/*
>  	 * Percpu reader mode:
> @@ -157,7 +157,6 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
>  
>  		smp_mb();
>  
> -		old = atomic_read(&lock->state);

Nope, this is wrong. That smp_mb() is there for a reason.

>  		ret = !(old & l[type].lock_fail);
>  
>  		this_cpu_sub(*lock->readers, !ret);
> @@ -182,7 +181,6 @@ static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
>  				ret = -1 - SIX_LOCK_read;
>  		}
>  	} else {
> -		old = atomic_read(&lock->state);
>  		do {
>  			ret = !(old & l[type].lock_fail);
>  			if (!ret || (type == SIX_LOCK_write && !try)) {
> -- 
> 2.34.1
> 

Re: [PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Hongbo Li]

On 2024/4/25 11:34, Kent Overstreet wrote:
> On Fri, Apr 19, 2024 at 03:48:50PM +0800, Hongbo Li wrote:
>> [BUG]
>> When compiling the bcachefs-tools, the following compilation warning
>> is reported:
>>      libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
>>      libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>>        915 |  snapshot->v.tree = cpu_to_le32(tree_id);
>>      libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
>>        903 |  u32 tree_id;
>>         |      ^~~~~~~
>>
>> [CAUSE]
>> This is a false alert, because @tree_id is changed in
>> bch2_snapshot_tree_create after it returns 0. And if this function
>> returns other value, @tree_id wouldn't be used. Thus there should
>> be nothing wrong in logical.
>>
>> [FIX]
>> Although the report itself is a false alert, we can still make it more
>> explicit by:
>>    - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
>>    - initialize @tree_id to U32_MAX
>>    - add extra WARN_ON_ONCE to make sure @tree_id is updated
>>
>> Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
>> Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
>> ---
>>   fs/bcachefs/snapshot.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
>> index 0b26dee17a5a..cad3408903b2 100644
>> --- a/fs/bcachefs/snapshot.c
>> +++ b/fs/bcachefs/snapshot.c
>> @@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
>>   static int bch2_snapshot_tree_create(struct btree_trans *trans,
>>   				u32 root_id, u32 subvol_id, u32 *tree_id)
>>   {
>> +	WARN_ON_ONCE(!tree_id);
> 
> There's no point checking for a null pointer like that; the oops from a
> null ptr deref gives us exactly the same information.
> 
>>   	struct bkey_i_snapshot_tree *n_tree =
>>   		__bch2_snapshot_tree_create(trans);
>>   
>> @@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>>   	if (bch2_snapshot_equiv(c, id))
>>   		return 0;
>>   
>> -	u32 tree_id;
>> +	u32 tree_id = U32_MAX;
> 
> Just initialize it to 0. 0 is an invalid tree ID, so it'll be caught by
> snapshot_tree_invalid() if it's not set.
> 
>>   	int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
>>   	if (ret)
>>   		return ret;
>> @@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>>   	if (ret)
>>   		return ret;
>>   
>> +	/* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */
>> +	WARN_ON_ONCE(tree_id == U32_MAX);
> 
> Nix this as well.
This is to ensure that the tree_id is modified.
> 
>>   	bkey_snapshot_init(&snapshot->k_i);
>>   	snapshot->k.p		= POS(0, id);
>>   	snapshot->v.tree	= cpu_to_le32(tree_id);
>> -- 
>> 2.34.1
>>

Re: [PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Kent Overstreet]

On Thu, Apr 25, 2024 at 11:55:02AM +0800, Hongbo Li wrote:
> 
> 
> On 2024/4/25 11:34, Kent Overstreet wrote:
> > On Fri, Apr 19, 2024 at 03:48:50PM +0800, Hongbo Li wrote:
> > > [BUG]
> > > When compiling the bcachefs-tools, the following compilation warning
> > > is reported:
> > >      libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
> > >      libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> > >        915 |  snapshot->v.tree = cpu_to_le32(tree_id);
> > >      libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
> > >        903 |  u32 tree_id;
> > >         |      ^~~~~~~
> > > 
> > > [CAUSE]
> > > This is a false alert, because @tree_id is changed in
> > > bch2_snapshot_tree_create after it returns 0. And if this function
> > > returns other value, @tree_id wouldn't be used. Thus there should
> > > be nothing wrong in logical.
> > > 
> > > [FIX]
> > > Although the report itself is a false alert, we can still make it more
> > > explicit by:
> > >    - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
> > >    - initialize @tree_id to U32_MAX
> > >    - add extra WARN_ON_ONCE to make sure @tree_id is updated
> > > 
> > > Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
> > > Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
> > > ---
> > >   fs/bcachefs/snapshot.c | 5 ++++-
> > >   1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
> > > index 0b26dee17a5a..cad3408903b2 100644
> > > --- a/fs/bcachefs/snapshot.c
> > > +++ b/fs/bcachefs/snapshot.c
> > > @@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
> > >   static int bch2_snapshot_tree_create(struct btree_trans *trans,
> > >   				u32 root_id, u32 subvol_id, u32 *tree_id)
> > >   {
> > > +	WARN_ON_ONCE(!tree_id);
> > 
> > There's no point checking for a null pointer like that; the oops from a
> > null ptr deref gives us exactly the same information.
> > 
> > >   	struct bkey_i_snapshot_tree *n_tree =
> > >   		__bch2_snapshot_tree_create(trans);
> > > @@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
> > >   	if (bch2_snapshot_equiv(c, id))
> > >   		return 0;
> > > -	u32 tree_id;
> > > +	u32 tree_id = U32_MAX;
> > 
> > Just initialize it to 0. 0 is an invalid tree ID, so it'll be caught by
> > snapshot_tree_invalid() if it's not set.
> > 
> > >   	int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
> > >   	if (ret)
> > >   		return ret;
> > > @@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
> > >   	if (ret)
> > >   		return ret;
> > > +	/* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */
> > > +	WARN_ON_ONCE(tree_id == U32_MAX);
> > 
> > Nix this as well.
> This is to ensure that the tree_id is modified.

It's still not a good assertion; it's checking something simple and
purely local, and we've got other checks that will fire later - it's not
guarding against undefined behaviour.

Re: [PATCH 1/2] bcachefs: eliminate the uninitialized compilation warning in bch2_reconstruct_snapshots

[Hongbo Li]

On 2024/4/26 1:08, Kent Overstreet wrote:
> On Thu, Apr 25, 2024 at 11:55:02AM +0800, Hongbo Li wrote:
>>
>>
>> On 2024/4/25 11:34, Kent Overstreet wrote:
>>> On Fri, Apr 19, 2024 at 03:48:50PM +0800, Hongbo Li wrote:
>>>> [BUG]
>>>> When compiling the bcachefs-tools, the following compilation warning
>>>> is reported:
>>>>       libbcachefs/snapshot.c: In function ‘bch2_reconstruct_snapshots’:
>>>>       libbcachefs/snapshot.c:915:19: warning: ‘tree_id’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>>>>         915 |  snapshot->v.tree = cpu_to_le32(tree_id);
>>>>       libbcachefs/snapshot.c:903:6: note: ‘tree_id’ was declared here
>>>>         903 |  u32 tree_id;
>>>>          |      ^~~~~~~
>>>>
>>>> [CAUSE]
>>>> This is a false alert, because @tree_id is changed in
>>>> bch2_snapshot_tree_create after it returns 0. And if this function
>>>> returns other value, @tree_id wouldn't be used. Thus there should
>>>> be nothing wrong in logical.
>>>>
>>>> [FIX]
>>>> Although the report itself is a false alert, we can still make it more
>>>> explicit by:
>>>>     - check the input parameter 'u32 *tree_id' with WARN_ON_ONCE
>>>>     - initialize @tree_id to U32_MAX
>>>>     - add extra WARN_ON_ONCE to make sure @tree_id is updated
>>>>
>>>> Fixes: a292be3b68f3 ("bcachefs: Reconstruct missing snapshot nodes")
>>>> Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
>>>> ---
>>>>    fs/bcachefs/snapshot.c | 5 ++++-
>>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
>>>> index 0b26dee17a5a..cad3408903b2 100644
>>>> --- a/fs/bcachefs/snapshot.c
>>>> +++ b/fs/bcachefs/snapshot.c
>>>> @@ -78,6 +78,7 @@ __bch2_snapshot_tree_create(struct btree_trans *trans)
>>>>    static int bch2_snapshot_tree_create(struct btree_trans *trans,
>>>>    				u32 root_id, u32 subvol_id, u32 *tree_id)
>>>>    {
>>>> +	WARN_ON_ONCE(!tree_id);
>>>
>>> There's no point checking for a null pointer like that; the oops from a
>>> null ptr deref gives us exactly the same information.
>>>
>>>>    	struct bkey_i_snapshot_tree *n_tree =
>>>>    		__bch2_snapshot_tree_create(trans);
>>>> @@ -900,7 +901,7 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>>>>    	if (bch2_snapshot_equiv(c, id))
>>>>    		return 0;
>>>> -	u32 tree_id;
>>>> +	u32 tree_id = U32_MAX;
>>>
>>> Just initialize it to 0. 0 is an invalid tree ID, so it'll be caught by
>>> snapshot_tree_invalid() if it's not set.
>>>
>>>>    	int ret = bch2_snapshot_tree_create(trans, id, 0, &tree_id);
>>>>    	if (ret)
>>>>    		return ret;
>>>> @@ -910,6 +911,8 @@ static int check_snapshot_exists(struct btree_trans *trans, u32 id)
>>>>    	if (ret)
>>>>    		return ret;
>>>> +	/* bch2_snapshot_tree_create returned 0, @tree_id must be updated. */
>>>> +	WARN_ON_ONCE(tree_id == U32_MAX);
>>>
>>> Nix this as well.
>> This is to ensure that the tree_id is modified.
> 
> It's still not a good assertion; it's checking something simple and
> purely local, and we've got other checks that will fire later - it's not
> guarding against undefined behaviour.
> 
Thank you, I will revise the code based on your reviews.