On 2023/8/3 6:40, Paul E. McKenney wrote:
> On Wed, Aug 02, 2023 at 09:09:18PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
>> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>>
>> When a structure containing an RCU callback rhp is (incorrectly) freed
>> and reallocated after rhp is passed to call_rcu(), it is not unusual for
>> rhp->func to be set to NULL. This defeats the debugging prints used by
>> __call_rcu_common() in kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y,
>> which expect to identify the offending code using the identity of this
>> function.
>>
>> And in kernels build without CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, things
>> are even worse, as can be seen from this splat:
>>
>> Unable to handle kernel NULL pointer dereference at virtual address 0
>> ... ...
>> PC is at 0x0
>> LR is at rcu_do_batch+0x1c0/0x3b8
>> ... ...
>> (rcu_do_batch) from (rcu_core+0x1d4/0x284)
>> (rcu_core) from (__do_softirq+0x24c/0x344)
>> (__do_softirq) from (__irq_exit_rcu+0x64/0x108)
>> (__irq_exit_rcu) from (irq_exit+0x8/0x10)
>> (irq_exit) from (__handle_domain_irq+0x74/0x9c)
>> (__handle_domain_irq) from (gic_handle_irq+0x8c/0x98)
>> (gic_handle_irq) from (__irq_svc+0x5c/0x94)
>> (__irq_svc) from (arch_cpu_idle+0x20/0x3c)
>> (arch_cpu_idle) from (default_idle_call+0x4c/0x78)
>> (default_idle_call) from (do_idle+0xf8/0x150)
>> (do_idle) from (cpu_startup_entry+0x18/0x20)
>> (cpu_startup_entry) from (0xc01530)
>>
>> This commit therefore adds calls to mem_dump_obj(rhp) to output some
>> information, for example:
>>
>> slab kmalloc-256 start ffff410c45019900 pointer offset 0 size 256
>>
>> This provides the rough size of the memory block and the offset of the
>> rcu_head structure, which as least provides at least a few clues to help
>> locate the problem. If the problem is reproducible, additional slab
>> debugging can be enabled, for example, CONFIG_DEBUG_SLAB=y, which can
>> provide significantly more information.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>
> Looks plausible, thank you!
>
> What did you do to test this?
This test is easier. I wrote a simple one myself.
static struct my_rcu_node *my_node;
static bool test_kmem_dump_obj(void)
{
void *p;
if (kmem_dump_obj(NULL))
return false;
if (kmem_dump_obj((void *)(PAGE_SIZE / 2)))
return false;
if (kmem_dump_obj((void *)(PAGE_SIZE - 1)))
return false;
if (kmem_dump_obj((void *)PAGE_SIZE))
return false;
if (kmem_dump_obj(&my_node))
return false;
p = vmalloc(0x100000);
WARN_ON(!p);
if (kmem_dump_obj(p)) {
vfree(p);
return false;
}
vfree(p);
p = kmalloc(0x100, GFP_KERNEL);
WARN_ON(!p);
if (!kmem_dump_obj(p)) {
kfree(p);
return false;
}
if (kmem_dump_obj((void *)(((unsigned long)p << 4) >> 4))) {
kfree(p);
return false;
}
kfree(p);
return true;
}
static int tst_proc_show(struct seq_file *m, void *v)
{
if (!test_kmem_dump_obj()) {
seq_printf(m, "test_kmem_dump_obj failed\n");
return 0;
}
my_node = kmalloc(sizeof(*my_node), GFP_KERNEL);
if (!my_node) {
seq_printf(m, "kmalloc failed\n");
return 0;
}
call_rcu(&my_node->node, my_rcu_cb);
my_node->node.func = NULL;
return 0;
}
>
> One option is the object_debug module parameter to rcutorture, which is
> described here: https://paulmck.livejournal.com/61432.html
OK, thanks for your info. I'll study the RCU self-test program rcutorture later.
>
>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> Not a big problem, but not a good habit to get into... I add my own
> Signed-off-by when I pull patches into my tree. Or if you are thinking
> in terms of sending this to mainline using some other path, when I am
> good with it, I would give you a tag to use.
Oh, Sorry. It seems that I forgot to delete your Signed-off-by in v2.
Oops! you reminded me once before. After v1, you helped modify the
description and pull it into your tree. I got it from 'dev' branch.
>
> So were you looking for me to take these two patches?
Yes, it could be quicker. Of course, I can wait for patch 1/2 upstream,
then repost patch 2/2. In fact, I also want to dump part of the slab
object, I've already written the code. In order not to affect the current
user of mem_dump_obj(), a new parameter need to be added to kmem_dump_obj().
I will post v5 with this patch later.
>
> Thanx, Paul
>
>> ---
>> kernel/rcu/rcu.h | 7 +++++++
>> kernel/rcu/srcutiny.c | 1 +
>> kernel/rcu/srcutree.c | 1 +
>> kernel/rcu/tasks.h | 1 +
>> kernel/rcu/tiny.c | 1 +
>> kernel/rcu/tree.c | 1 +
>> 6 files changed, 12 insertions(+)
>>
>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>> index d1dcb09750efbd6..bc81582238b9846 100644
>> --- a/kernel/rcu/rcu.h
>> +++ b/kernel/rcu/rcu.h
>> @@ -10,6 +10,7 @@
>> #ifndef __LINUX_RCU_H
>> #define __LINUX_RCU_H
>>
>> +#include <linux/slab.h>
>> #include <trace/events/rcu.h>
>>
>> /*
>> @@ -248,6 +249,12 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
>> }
>> #endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
>>
>> +static inline void debug_rcu_head_callback(struct rcu_head *rhp)
>> +{
>> + if (unlikely(!rhp->func))
>> + kmem_dump_obj(rhp);
>> +}
>> +
>> extern int rcu_cpu_stall_suppress_at_boot;
>>
>> static inline bool rcu_stall_is_suppressed_at_boot(void)
>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>> index 336af24e0fe358a..c38e5933a5d6937 100644
>> --- a/kernel/rcu/srcutiny.c
>> +++ b/kernel/rcu/srcutiny.c
>> @@ -138,6 +138,7 @@ void srcu_drive_gp(struct work_struct *wp)
>> while (lh) {
>> rhp = lh;
>> lh = lh->next;
>> + debug_rcu_head_callback(rhp);
>> local_bh_disable();
>> rhp->func(rhp);
>> local_bh_enable();
>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>> index f1a905200fc2f79..833a8f848a90ae6 100644
>> --- a/kernel/rcu/srcutree.c
>> +++ b/kernel/rcu/srcutree.c
>> @@ -1710,6 +1710,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
>> rhp = rcu_cblist_dequeue(&ready_cbs);
>> for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
>> debug_rcu_head_unqueue(rhp);
>> + debug_rcu_head_callback(rhp);
>> local_bh_disable();
>> rhp->func(rhp);
>> local_bh_enable();
>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>> index 7294be62727b12c..148ac6a464bfb12 100644
>> --- a/kernel/rcu/tasks.h
>> +++ b/kernel/rcu/tasks.h
>> @@ -538,6 +538,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
>> raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
>> len = rcl.len;
>> for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
>> + debug_rcu_head_callback(rhp);
>> local_bh_disable();
>> rhp->func(rhp);
>> local_bh_enable();
>> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
>> index 42f7589e51e09e7..fec804b7908032d 100644
>> --- a/kernel/rcu/tiny.c
>> +++ b/kernel/rcu/tiny.c
>> @@ -97,6 +97,7 @@ static inline bool rcu_reclaim_tiny(struct rcu_head *head)
>>
>> trace_rcu_invoke_callback("", head);
>> f = head->func;
>> + debug_rcu_head_callback(head);
>> WRITE_ONCE(head->func, (rcu_callback_t)0L);
>> f(head);
>> rcu_lock_release(&rcu_callback_map);
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 7c79480bfaa04e4..927c5ba0ae42269 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -2135,6 +2135,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>> trace_rcu_invoke_callback(rcu_state.name, rhp);
>>
>> f = rhp->func;
>> + debug_rcu_head_callback(rhp);
>> WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
>> f(rhp);
>>
>> --
>> 2.34.1
>>
> .
>
--
Regards,
Zhen Lei
