linux-kernel - Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic op overhead

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Message-ID: <18893.1299607373@redhat.com>
Date:	Tue, 08 Mar 2011 18:02:53 +0000
From:	David Howells <dhowells@...hat.com>
To:	Tony Jones <tonyj@...e.de>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-audit@...hat.com, Eric Paris <eparis@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic op overhead

Tony Jones <tonyj@...e.de> wrote:

> Commit c69e8d9c01db added calls to get_task_cred and put_cred in
> audit_filter_rules.  Profiling with a large number of audit rules active on
> the exit chain shows that we are spending upto 48% in this routine for
> syscall intensive tests, most of which is in the atomic ops.
> 
> The following patch acquires the cred if a rule requires it.  In our
> particular case above, most rules had no cred requirement and this dropped
> the time spent in audit_filter_rules down to ~12%.  An alternative would be
> for the caller to acquire the cred just once for the whole chain and pass
> into audit_filter_rules.  I can create an alternate patch doing this if
> required.

There's no actual need to get a ref on the named task's creds.

If tsk == current, no locking is needed at all.

If tsk != current, the RCU read lock is sufficient.  See task_cred_xxx() in
include/linux/cred.h.

Hmmm...  I wonder...  The audit filter uses tsk->real_cred, but is that
correct?  Should it be using tsk->cred?  And is tsk always going to be
current?

> +	const struct cred *cred=NULL;

Binary operators like '=' should have a space on each side.

David
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