When booting on 32-bit machines (seen on OpenRISC) I saw this warning
with CONFIG_DEBUG_MUTEXES turned on.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at kernel/locking/mutex.c:1242 __mutex_unlock_slowpath+0x328/0x3ec
DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current)
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.8.0-rc1-simple-smp-00005-g2864e2171db4-dirty #179
Call trace:
[<(ptrval)>] dump_stack+0x34/0x48
[<(ptrval)>] __warn+0x104/0x158
[<(ptrval)>] ? __mutex_unlock_slowpath+0x328/0x3ec
[<(ptrval)>] warn_slowpath_fmt+0x7c/0x94
[<(ptrval)>] __mutex_unlock_slowpath+0x328/0x3ec
[<(ptrval)>] mutex_unlock+0x18/0x28
[<(ptrval)>] __cpuhp_setup_state_cpuslocked.part.0+0x29c/0x2f4
[<(ptrval)>] ? page_alloc_cpu_dead+0x0/0x30
[<(ptrval)>] ? start_kernel+0x0/0x684
[<(ptrval)>] __cpuhp_setup_state+0x4c/0x5c
[<(ptrval)>] page_alloc_init+0x34/0x68
[<(ptrval)>] ? start_kernel+0x1a0/0x684
[<(ptrval)>] ? early_init_dt_scan_nodes+0x60/0x70
irq event stamp: 0
I traced this to kernel/locking/mutex.c storing 3 bits of MUTEX_FLAGS in
the task_struct pointer (mutex.owner). There is a comment saying that
task_structs are always aligned to L1_CACHE_BYTES. This is not true for
the init_task.
On 64-bit machines this is not a problem because symbol addresses are
naturally aligned to 64-bits providing 3 bits for MUTEX_FLAGS. Howerver,
for 32-bit machines the symbol address only has 2 bits available.
Fix this by setting init_task alignment to at least L1_CACHE_BYTES.
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Stafford Horne <[email protected]>
---
init/init_task.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/init/init_task.c b/init/init_task.c
index 15089d15010a..d2d2af018d0d 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -64,6 +64,8 @@ unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)]
struct task_struct init_task
#ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
__init_task_data
+#else
+ __aligned(L1_CACHE_BYTES)
#endif
= {
#ifdef CONFIG_THREAD_INFO_IN_TASK
--
2.26.2
On Fri, Jun 26, 2020 at 05:44:09AM +0900, Stafford Horne wrote:
> When booting on 32-bit machines (seen on OpenRISC) I saw this warning
> with CONFIG_DEBUG_MUTEXES turned on.
> I traced this to kernel/locking/mutex.c storing 3 bits of MUTEX_FLAGS in
> the task_struct pointer (mutex.owner). There is a comment saying that
> task_structs are always aligned to L1_CACHE_BYTES. This is not true for
> the init_task.
>
> On 64-bit machines this is not a problem because symbol addresses are
> naturally aligned to 64-bits providing 3 bits for MUTEX_FLAGS. Howerver,
> for 32-bit machines the symbol address only has 2 bits available.
>
> Fix this by setting init_task alignment to at least L1_CACHE_BYTES.
Whoopsie, sorry about that.
> Cc: Peter Zijlstra <[email protected]>
> Signed-off-by: Stafford Horne <[email protected]>
> ---
> init/init_task.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/init/init_task.c b/init/init_task.c
> index 15089d15010a..d2d2af018d0d 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -64,6 +64,8 @@ unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)]
> struct task_struct init_task
> #ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
> __init_task_data
> +#else
> + __aligned(L1_CACHE_BYTES)
> #endif
Why make this conditional? task_struct_cachep (in kernel/fork.c) has
max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN) alignment, so this really
should be aligned on L1_CACHE_BYTES at least.
On Fri, Jun 26, 2020 at 01:12:08PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 26, 2020 at 05:44:09AM +0900, Stafford Horne wrote:
> > When booting on 32-bit machines (seen on OpenRISC) I saw this warning
> > with CONFIG_DEBUG_MUTEXES turned on.
>
> > I traced this to kernel/locking/mutex.c storing 3 bits of MUTEX_FLAGS in
> > the task_struct pointer (mutex.owner). There is a comment saying that
> > task_structs are always aligned to L1_CACHE_BYTES. This is not true for
> > the init_task.
> >
> > On 64-bit machines this is not a problem because symbol addresses are
> > naturally aligned to 64-bits providing 3 bits for MUTEX_FLAGS. Howerver,
> > for 32-bit machines the symbol address only has 2 bits available.
> >
> > Fix this by setting init_task alignment to at least L1_CACHE_BYTES.
>
> Whoopsie, sorry about that.
>
> > Cc: Peter Zijlstra <[email protected]>
> > Signed-off-by: Stafford Horne <[email protected]>
> > ---
> > init/init_task.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/init/init_task.c b/init/init_task.c
> > index 15089d15010a..d2d2af018d0d 100644
> > --- a/init/init_task.c
> > +++ b/init/init_task.c
> > @@ -64,6 +64,8 @@ unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)]
> > struct task_struct init_task
> > #ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
> > __init_task_data
> > +#else
> > + __aligned(L1_CACHE_BYTES)
> > #endif
>
> Why make this conditional? task_struct_cachep (in kernel/fork.c) has
> max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN) alignment, so this really
> should be aligned on L1_CACHE_BYTES at least.
I think we can make this unconditional. The only reason I used the condition is
because the only architecture that sets CONFIG_ARCH_TASK_STRUCT_ON_STACK is ia64
which already has 64-bit alignment.
I can change it to (not incorporating max_t(ARCH_MIN_TASKALIGN) as not sure how):
diff --git a/init/init_task.c b/init/init_task.c
index 15089d15010a..ab6173f8e6a8 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -65,6 +65,7 @@ struct task_struct init_task
#ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
__init_task_data
#endif
+ __aligned(L1_CACHE_BYTES)
= {
#ifdef CONFIG_THREAD_INFO_IN_TASK
.thread_info = INIT_THREAD_INFO(init_task),
-Stafford
On Fri, Jun 26, 2020 at 08:45:22PM +0900, Stafford Horne wrote:
> I think we can make this unconditional. The only reason I used the condition is
> because the only architecture that sets CONFIG_ARCH_TASK_STRUCT_ON_STACK is ia64
> which already has 64-bit alignment.
>
> I can change it to (not incorporating max_t(ARCH_MIN_TASKALIGN) as not sure how):
The only arch that has ARCH_MIN_TASKALIGN larger than L1_CACHE_SIZE is
some daft x86 config and it shouldn't be fatal to them.
So with this:
Acked-by: Peter Zijlstra (Intel) <[email protected]>
> diff --git a/init/init_task.c b/init/init_task.c
> index 15089d15010a..ab6173f8e6a8 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -65,6 +65,7 @@ struct task_struct init_task
> #ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
> __init_task_data
> #endif
> + __aligned(L1_CACHE_BYTES)
> = {
> #ifdef CONFIG_THREAD_INFO_IN_TASK
> .thread_info = INIT_THREAD_INFO(init_task),
>
> -Stafford