fs/seq_file.c: simplify seq_file iteration code and interface
The documentation for seq_file suggests that it is necessary to be able to
move the iterator to a given offset, however that is not the case. If the
iterator is stored in the private data and is stable from one read()
syscall to the next, it is only necessary to support first/next
interactions. Implementing this in a client is a little clumsy.
- if ->start() is given a pos of zero, it should go to start of
sequence.
- if ->start() is given the name pos that was given to the most recent
next() or start(), it should restore the iterator to state just before
that last call
- if ->start is given another number, it should set the iterator one
beyond the start just before the last ->start or ->next call.
Also, the documentation says that the implementation can interpret the pos
however it likes (other than zero meaning start), but seq_file increments
the pos sometimes which does impose on the implementation.
This patch simplifies the interface for first/next iteration and
simplifies the code, while maintaining complete backward compatability.
Now:
- if ->start() is given a pos of zero, it should return an iterator
placed at the start of the sequence
- if ->start() is given a non-zero pos, it should return the iterator
in the same state it was after the last ->start or ->next.
This is particularly useful for interators which walk the multiple chains
in a hash table, e.g. using rhashtable_walk*. See fs/gfs2/glock.c and
drivers/staging/lustre/lustre/llite/vvp_dev.c
A large part of achieving this is to *always* call ->next after ->show has
successfully stored all of an entry in the buffer. Never just increment
the index instead. Also:
- always pass &m->index to ->start() and ->next(), never a temp
variable
- don't clear ->from when ->count is zero, as ->from is dead when
->count is zero.
Some ->next functions do not increment *pos when they return NULL. To
maintain compatability with this, we still need to increment m->index in
one place, if ->next didn't increment it. Note that such ->next functions
are buggy and should be fixed. A simple demonstration is
dd if=/proc/swaps bs=1000 skip=1
Choose any block size larger than the size of /proc/swaps. This will
always show the whole last line of /proc/swaps.
This patch doesn't work around buggy next() functions for this case.
Signed-off-by: NeilBrown <neilb@suse.com>
Acked-by: Jonathan Corbet <corbet@lwn.net> [docs]
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2 files changed
