Hi,

the kernel contains an optimization that skips the linked list walk in 
get_tgid_list for the common case of sequential accesses.
Unfortunately the optimization is buggy (missing NULL pointer check for 
the result of find_task_by_pid) and broken (actually - broken twice: the 
tgid value that is stored in f_version is always 0 because tgid is 
overwritten when the string is created and additionally the common case 
is not filldir < 0, it's running out of nr_tgids).

The attached patch fixes these bugs.

Roger: could you run your 100k processes test against a kernel with this 
fix applied? I'm just interested how much it helps. One obvious result 
are fewer getdents64 syscalls: instead of returning 20 entries (480 
bytes) the new code fills the user space buffer completely (42 
entries/syscall).

Andrew, could you add the patch to your -mm tree?

Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>