> Well looking a little more closely than I did last night it looks like
> access_process_vm (called from ptrace) can cause what amounts to a
> page fault at pretty arbitrary times.
It's also used for several /proc/<pid> files.
I remember that I got crashes with concurrent exec+cat
/proc/<pid>/cmdline until down(mmap_sem) was added into
setup_arg_pages().
> I'm actually a little curious what the big kernel lock in ptrace buys
> us. I suspect it could be a performance issue with user mode linux.
> Where you have multiple processes being ptraced at the same time.
I checked it a few months ago, and the lock is only required to prevent
multiple concurrent attaches to the same process and concorrent
ptrace/suid exec (in fs/exec.c, compute_creds)
--
Manfred
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