[PATCH] make rlimit settings per-process instead of per-thread

POSIX specifies that the limit settings provided by getrlimit/setrlimit are
shared by the whole process, not specific to individual threads.  This
patch changes the behavior of those calls to comply with POSIX.

I've moved the struct rlimit array from task_struct to signal_struct, as it
has the correct sharing properties.  (This reduces kernel memory usage per
thread in multithreaded processes by around 100/200 bytes for 32/64
machines respectively.)  I took a fairly minimal approach to the locking
issues with the newly shared struct rlimit array.  It turns out that all
the code that is checking limits really just needs to look at one word at a
time (one rlim_cur field, usually).  It's only the few places like
getrlimit itself (and fork), that require atomicity in accessing a whole
struct rlimit, so I just used a spin lock for them and no locking for most
of the checks.  If it turns out that readers of struct rlimit need more
atomicity where they are now cheap, or less overhead where they are now
atomic (e.g. fork), then seqcount is certainly the right thing to use for
them instead of readers using the spin lock.  Though it's in signal_struct,
I didn't use siglock since the access to rlimits never needs to disable
irqs and doesn't overlap with other siglock uses.  Instead of adding
something new, I overloaded task_lock(task->group_leader) for this; it is
used for other things that are not likely to happen simultaneously with
limit tweaking.  To me that seems preferable to adding a word, but it would
be trivial (and arguably cleaner) to add a separate lock for these users
(or e.g. just use seqlock, which adds two words but is optimal for readers).

Most of the changes here are just the trivial s/->rlim/->signal->rlim/. 

I stumbled across what must be a long-standing bug, in reparent_to_init.
It does:
	memcpy(current->rlim, init_task.rlim, sizeof(*(current->rlim)));
when surely it was intended to be:
	memcpy(current->rlim, init_task.rlim, sizeof(current->rlim));
As rlim is an array, the * in the sizeof expression gets the size of the
first element, so this just changes the first limit (RLIMIT_CPU).  This is
for kernel threads, where it's clear that resetting all the rlimits is what
you want.  With that fixed, the setting of RLIMIT_FSIZE in nfsd is
superfluous since it will now already have been reset to RLIM_INFINITY.

The other subtlety is removing:
	tsk->rlim[RLIMIT_CPU].rlim_cur = RLIM_INFINITY;
in exit_notify, which was to avoid a race signalling during self-reaping
exit.  As the limit is now shared, a dying thread should not change it for
others.  Instead, I avoid that race by checking current->state before the
RLIMIT_CPU check.  (Adding one new conditional in that path is now required
one way or another, since if not for this check there would also be a new
race with self-reaping exit later on clearing current->signal that would
have to be checked for.)

The one loose end left by this patch is with process accounting.
do_acct_process temporarily resets the RLIMIT_FSIZE limit while writing the
accounting record.  I left this as it was, but it is now changing a limit
that might be shared by other threads still running.  I left this in a
dubious state because it seems to me that processing accounting may already
be more generally a dubious state when it comes to NPTL threads.  I would
think you would want one record per process, with aggregate data about all
threads that ever lived in it, not a separate record for each thread.
I don't use process accounting myself, but if anyone is interested in
testing it out I could provide a patch to change it this way.

One final note, this is not 100% to POSIX compliance in regards to rlimits.
POSIX specifies that RLIMIT_CPU refers to a whole process in aggregate, not
to each individual thread.  I will provide patches later on to achieve that
change, assuming this patch goes in first.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index aee2696c30b1ef..5cfcd4b68cb3f9 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -145,7 +145,7 @@ static struct inode *mqueue_get_inode(struct super_block *sb, int mode,
 			spin_lock(&mq_lock);
 			if (u->mq_bytes + mq_bytes < u->mq_bytes ||
 		 	    u->mq_bytes + mq_bytes >
-			    p->rlim[RLIMIT_MSGQUEUE].rlim_cur) {
+			    p->signal->rlim[RLIMIT_MSGQUEUE].rlim_cur) {
 				spin_unlock(&mq_lock);
 				goto out_inode;
 			}