lib/Kconfig.debug | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
From: "J. R. Okajima" <hooanon05g@gmail.com>
CONFIG_LOCKDEP_CHAINS_BITS value decides the size of chain_hlocks[] in
kernel/locking/lockdep.c, and it is checked by add_chain_cache() with
BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
This patch is just to silence BUILD_BUG_ON().
See-also: https://lore.kernel.org/all/30795.1620913191@jrobl/
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
[cmllamas: fix minor checkpatch issues in commit log]
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
lib/Kconfig.debug | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a30c03a66172..04668248225c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1515,7 +1515,7 @@ config LOCKDEP_BITS
config LOCKDEP_CHAINS_BITS
int "Bitsize for MAX_LOCKDEP_CHAINS"
depends on LOCKDEP && !LOCKDEP_SMALL
- range 10 30
+ range 10 21
default 16
help
Try increasing this value if you hit "BUG: MAX_LOCKDEP_CHAINS too low!" message.
--
2.45.2.1089.g2a221341d9-goog
On Tue, 23 Jul 2024 16:40:17 +0000 Carlos Llamas <cmllamas@google.com> wrote: > From: "J. R. Okajima" <hooanon05g@gmail.com> > > CONFIG_LOCKDEP_CHAINS_BITS value decides the size of chain_hlocks[] in > kernel/locking/lockdep.c, and it is checked by add_chain_cache() with > BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)); > This patch is just to silence BUILD_BUG_ON(). > > ... > > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -1515,7 +1515,7 @@ config LOCKDEP_BITS > config LOCKDEP_CHAINS_BITS > int "Bitsize for MAX_LOCKDEP_CHAINS" > depends on LOCKDEP && !LOCKDEP_SMALL > - range 10 30 > + range 10 21 > default 16 > help > Try increasing this value if you hit "BUG: MAX_LOCKDEP_CHAINS too low!" message. checking your homework... With LOCKDEP_CHAINS_BITS == 21: #define MAX_LOCKDEP_CHAINS_BITS CONFIG_LOCKDEP_CHAINS_BITS gives MAX_LOCKDEP_CHAINS_BITS == 21 #define MAX_LOCKDEP_CHAINS (1UL << MAX_LOCKDEP_CHAINS_BITS) gives MAX_LOCKDEP_CHAINS == (1UL << 21) #define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5) gives MAX_LOCKDEP_CHAIN_HLOCKS = 5 * (1UL << 21) static u16 chain_hlocks[MAX_LOCKDEP_CHAIN_HLOCKS]; gives ARRAY_SIZE(chain_hlocks) == 5 * (1UL << 21) so BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)); ie, BUILD_BUG_ON((1UL << 24) <= 5 * (1UL << 21)); is OK, whereas BUILD_BUG_ON((1UL << 24) <= 5 * (1UL << 22)); will bug out. So LGTM, I'll add it to mm.git. btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd. What's a bitsize? Maybe "bit shift count for..." or such.
On Wed, Jul 31, 2024 at 04:48:23PM -0700, Andrew Morton wrote: > btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd. What's a > bitsize? Maybe "bit shift count for..." or such. Yeah, maybe a _SHIFT suffix would have been better fit for these configs instead of _BITS. Similar to PAGE_SHIFT or NODES_SHIFT. Anyways, I can send a patch to improve the help text. How about something like: "Size for ... (as a power of 2)" I'll also send a separate patch to fix the rest of the upper limits. Any of the (1 << 30) shifts allocates static arrays that blow past the .bss segment. Just as originally reporeted by J. R. Okajima. -- Carlos Llamas
On Mon, Aug 05, 2024 at 07:58:07PM +0000, Carlos Llamas wrote: > On Wed, Jul 31, 2024 at 04:48:23PM -0700, Andrew Morton wrote: > > btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd. What's a > > bitsize? Maybe "bit shift count for..." or such. > > Yeah, maybe a _SHIFT suffix would have been better fit for these configs > instead of _BITS. Similar to PAGE_SHIFT or NODES_SHIFT. Anyways, I can > send a patch to improve the help text. How about something like: > "Size for ... (as a power of 2)" > > I'll also send a separate patch to fix the rest of the upper limits. Any > of the (1 << 30) shifts allocates static arrays that blow past the .bss > segment. Just as originally reporeted by J. R. Okajima. Ok, I've sent a patchset for these bits at: https://lore.kernel.org/all/20240806010128.402852-1-cmllamas@google.com/ Thanks, Carlos Llamas
On Wed, Jul 31, 2024 at 04:48:23PM -0700, Andrew Morton wrote: [...] > so > > BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)); > ie, BUILD_BUG_ON((1UL << 24) <= 5 * (1UL << 21)); > > is OK, whereas > > BUILD_BUG_ON((1UL << 24) <= 5 * (1UL << 22)); > > will bug out. So LGTM, I'll add it to mm.git. > Right. I ran into the BUILD_BUG_ON() while trying to max out LOCKDEP_CHAINS_BITS. I initially suspected the assert was incorrect as the static array is being indexed as chain_hlocks[base + depth], which according to the bitfileds in 'struct lock_chain' should likely be a 30 bit shift instead: unsigned int irq_contex : 2, depth : 6, base : 24; In practice though, using 1UL << 30 will blow up the bss section. This is also true for the _any_ of the CONFIG_LOCKDEP_*_BITS. As they are all shifts to determine the size of static arrays. I simply dug up this patch from J.R. which avoids the BUILD_BUG_ON(), but perhaps someone should limit the rest of the configs? In practice, nobody should be using these 30 bit shifts. > btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd. What's a > bitsize? Maybe "bit shift count for..." or such. Indeed that is odd. I'm also not sure what to make of the "*5" magic number. I suppose it could be the typical lock depth? I could try to clarify these points, if no one with more insight wants to do it. -- Carlos Llamas
Carlos Llamas: > I simply dug up this patch from J.R. which avoids the BUILD_BUG_ON(), > but perhaps someone should limit the rest of the configs? In practice, > nobody should be using these 30 bit shifts. I posted the patch in 2021. It was against the commit in v5.13-rc1, 5dc33592e9553 2021-04-05 lockdep: Allow tuning tracing capacity constants. It is a little surprise for me that you could pick up such old post. As I wrote in the old post https://lore.kernel.org/all/30795.1620913191@jrobl/ I don't know what this 'multiply by 5' means and why ARRAY_SIZE(chain_hlocks) is limited to (1UL << 24) And the post is just to silence BUILD_BUG_ON(). There are a few more "range 10 30" in lib/Kconfig.debug. > > btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd. What's a > > bitsize? Maybe "bit shift count for..." or such. > > Indeed that is odd. I'm also not sure what to make of the "*5" magic > number. I suppose it could be the typical lock depth? I could try to > clarify these points, if no one with more insight wants to do it. Also the original text comes from the commit in v5.13-rc1. J. R. Okajima
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