Re: [PATCH v2 21/27] coresight: Convert driver messages to dev_dbg
From: Robin Murphy
Date: Wed May 02 2018 - 09:52:53 EST
On 02/05/18 04:55, Kim Phillips wrote:
On Tue, 1 May 2018 10:10:51 +0100
Suzuki K Poulose <suzuki.poulose@xxxxxxx> wrote:
Convert component enable/disable messages from dev_info to dev_dbg.
This is required to prevent LOCKDEP splats when operating in perf
mode where we could be called with locks held to enable a coresight
Can we see the splats? Doesn't lockdep turn itself off if it starts
triggering too many splats?
Without some very careful and robust reasoning for why the condition
being reported by lockdep could not actually occur in practice,
"avoiding the splats" is far, far less important than "avoiding the
potential deadlock that they are reporting".
path. If someone wants to really see the messages, they can always
enable it at runtime via dynamic_debug.
Won't the splats still occur when the messages are enabled with
dynamic_debug?
So in effect this patch only tries to mitigate the splats, all the
while making things harder for regular users that now have to recompile
their kernels, in exchange for a very small convenience for kernel
developers that happen to see a splat or two with DEBUG_LOCKDEP set?
FWIW, if "regular users" means people running distro kernels, then
chances are that they probably have DYNAMIC_DEBUG set already (100% of
my local sample of 2 - Ubuntu x86_64 and Arch aarch64 - certainly do).
Either way, though, this particular log spam really does only look
vaguely useful to people debugging the coresight stack itself, so anyone
going out of their way to turn it on has surely already gone beyond
regular use (even if they're just reproducing an issue with additional
logging at the request of kernel developers, rather than debugging it
themselves).
Reducing the scope for possible deadlock from the general case to just
debugging scenarios is certainly not a bad thing, but as you say I think
we need a closer look at the underlying issue to know whether even
dev_dbg() is wise.
Robin.