[Crash-utility] Traversing hlist_node linked structures

[Dave Anderson]

----- Original Message -----
> 
> 
> ----- Original Message -----
> > Hello,
> > 
> > I'd like to ask whether it's possible to traverse the chains of a
> > hashtable which is defined via the hashtable.h infrastructure? If
> > there is no support for this currently I guess crash has to be taught
> > to understand how to do the maths to extract the container structures
> > from embedded hlist_nodes, similar to how it does it for list_head
> > type of structs? And unless this is done doing the arithmetic manually
> > is the only way?
> > 
> > Regards,
> > Nikolay
> 
> You can easily traverse the chain of hlist_node structures linked from
> an hlist_head using the "list" command, but if you want to display the data
> structure that contains each hlist_node, it's typically a two-step process
> where you:
>  
>  (1) capture the list of hlist_node addresses in an output file, and then
>  (2) feed that list back to "struct -l offset", where the offset to the
>      hlist_node is supplied.
> 
> If you're lucky and the hlist_node happens to be the first member of the
> containing structure, you could use the "list -s" option directly.
> 
> An interesting option to "list [-s struct[.member[,member]] ..." might
> be the addition of a "-l offset", which would only be accepted/utilized
> if "list -s ..." is being used, i.e.:
> 
>   list [-s struct[.member[,member] [-l offset]]
> 
> Dave

Actually, for a hashtable headed up by a hlist_head, I think you should be able
to use the "list -H" option. 
 
For example, each of the kernel's pid_hash[] chains is headed by an hlist_head 
structure, which links a chain of "upid" structures:

  crash> whatis pid_hash
  struct hlist_head *pid_hash;
  crash> upid
  struct upid {
      int nr;
      struct pid_namespace *ns;
      struct hlist_node pid_chain;
  }
  SIZE: 32
  crash> 

Here, one of the pid_hash[] chains has 3 members:

  crash> p pid_hash[1630]
  $5 = {
    first = 0xffff88007786e7c0
  }
  crash> list 0xffff88007786e7c0
  ffff88007786e7c0
  ffff88022ff29240
  ffff88022da515c0
  crash>

Using the address of the hlist_head: 
  
  crash> p &pid_hash[1630]
  $4 = (struct hlist_head *) 0xffff88043ffb5a70
  crash>

I can dump the upid structures by passing that address to -H, along
with -s and -o:

  crash> list -H 0xffff88043ffb5a70 -s upid -o upid.pid_chain
  ffff88007786e7b0
  struct upid {
    nr = 58308, 
    ns = 0xffffffff819411a0 <init_pid_ns>, 
    pid_chain = {
      next = 0xffff88022ff29240, 
      pprev = 0xffff88043ffb5a70
    }
  }
  ffff88022ff29230
  struct upid {
    nr = 57787, 
    ns = 0xffffffff819411a0 <init_pid_ns>, 
    pid_chain = {
      next = 0xffff88022da515c0, 
      pprev = 0xffff88007786e7c0
    }
  }
  ffff88022da515b0
  struct upid {
    nr = 964, 
    ns = 0xffffffff819411a0 <init_pid_ns>, 
    pid_chain = {
      next = 0x0, 
      pprev = 0xffff88022ff29240
    }
  }
  crash> 

Dave