Re: no need to check for NULL before calling kfree() -fs/ext2/

[P Lavin]

Hi Jesper,
I'm sending this mail to mailing list coz in my company we have some
restrictions on o/g mails, Sorry for that...
Lemme ask u smthing, herez the code
    199     sndpkt = (RSI_sndpkt_t *) RSI_MALLOC(sizeof(RSI_sndpkt_t));
    200     sndpkt->buf_list = (RSI_buf_t *) RSI_MALLOC(sizeof(RSI_buf_t));
Here if malloc fails sndpkt->buf_list should be null right ?? & if i
proceed further ..

    201     sndpkt->buf_list->start_addr = buf;
    202     sndpkt->buf_list->length     = length;
Here itself this should crash right ?? But its not crashing here !!! Wt
was happening was

201 sndpkt->buf_list->start_addr = buf; was not getting initailised & wn
we try to access this variable latter
this was crashing.

Actally i'm not checking for return value from kmalloc thatz a mistake,
I'll fix this but why is it not crashing in line # 201 ??

Jesper Juhl wrote:

> On Wed, 30 Mar 2005, P Lavin wrote:
>
>  
>
> > Date: Wed, 30 Mar 2005 12:45:01 +0530
> > From: P Lavin <lavin.p@xxxxxxxxxxxxxxxxxx>
> > To: linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: no need to check for NULL before calling kfree() -fs/ext2/
> >
> > Hi,
> > In my wlan driver module, i allocated some memory using kmalloc in interrupt
> > context, this one failed but its not returning NULL , 
> >    
>
> kmalloc() should always return NULL if the allocation failed.
>
>
>  
>
> > so i was proceeding
> > further everything was going wrong... & finally the kernel crahed. Can any one
> > of you tell me why this is happening ? i cannot use GFP_KERNEL because i'm
> > calling this function from interrupt context & it may block. Any other
> >    
>
> If you need to allocate memory from interrupt context you should be using 
> GFP_ATOMIC (or, if possible, do the allocation earlier in a different 
> context).
>
>
>  
I'm using this flag only, this flag does not guarentee mem allocation,
right ??

> > solution for this ?? I'm concerned abt why kmalloc is not returning null if
> > its not a success ??
> >
> >    
> I have no explanation for that, are you sure that's really what's 
> happening?
>
>
>  
I'm not checking this , but my explanation is given above.

> > Is it not necessary to check for NULL before calling kfree() ??
> >    
>
> No, it is not nessesary to check for NULL before calling kfree() since 
> kfree() does    
>
> void kfree (const void *objp)
> {
> 	... 
>        if (!objp)
>                return;
> 	...
> }
>
> So, if you pass kfree() a NULL pointer it deals with it itself, you don't 
> need to check that explicitly before calling kfree() - that's redundant.
>
>
>  

Regs,
Lavin
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