Re: [V2 PATCH 2/3] kexec: Fix race between panic() and crash_kexec() called directly
Hi,
(2015/07/27 23:55), Michal Hocko wrote:
> On Mon 27-07-15 10:58:50, Hidehiro Kawai wrote:
> [...]
>> @@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void)
>>
>> void crash_kexec(struct pt_regs *regs)
>> {
>> +int old_cpu, this_cpu;
>> +
>> +/*
>> + * `old_cpu == -1' means we are the first comer and crash_kexec()
>> + * was called without entering panic().
>> + * `old_cpu == this_cpu' means crash_kexec() was called from panic().
>> + */
>> +this_cpu = raw_smp_processor_id();
>> +old_cpu = atomic_cmpxchg(&panicking_cpu, -1, this_cpu);
>> +if (old_cpu != -1 && old_cpu != this_cpu)
>> +return;
>> +
>> /* Take the kexec_mutex here to prevent sys_kexec_load
>> * running on one cpu from replacing the crash kernel
>> * we are using after a panic on a different cpu.
>> @@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs)
>> }
>> mutex_unlock(&kexec_mutex);
>> }
>> +
>> +/*
>> + * If we came here from panic(), we have to keep panicking_cpu
>> + * to prevent other cpus from entering panic(). Otherwise,
>> + * resetting it so that other cpus can enter panic()/crash_kexec().
>> + */
>> +if (old_cpu == this_cpu)
>> +atomic_set(&panicking_cpu, -1);
>
> This do the opposite what the comment says, wouldn't it? You should
> check old_cpu == -1.
Sorry, you are right. I performed same tests as for the
previous patch set, but I missed the test case for this
new logic.
> Also atomic_set doesn't imply memory barriers which
> might be a problem.
OK, I'll use atomic_xchg().
Regards,
--
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group
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Re: [V2 PATCH 2/3] kexec: Fix race between panic() and crash_kexec() called directly
On Mon 27-07-15 10:58:50, Hidehiro Kawai wrote:
[...]
> @@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void)
>
> void crash_kexec(struct pt_regs *regs)
> {
> + int old_cpu, this_cpu;
> +
> + /*
> + * `old_cpu == -1' means we are the first comer and crash_kexec()
> + * was called without entering panic().
> + * `old_cpu == this_cpu' means crash_kexec() was called from panic().
> + */
> + this_cpu = raw_smp_processor_id();
> + old_cpu = atomic_cmpxchg(&panicking_cpu, -1, this_cpu);
> + if (old_cpu != -1 && old_cpu != this_cpu)
> + return;
> +
> /* Take the kexec_mutex here to prevent sys_kexec_load
>* running on one cpu from replacing the crash kernel
>* we are using after a panic on a different cpu.
> @@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs)
> }
> mutex_unlock(&kexec_mutex);
> }
> +
> + /*
> + * If we came here from panic(), we have to keep panicking_cpu
> + * to prevent other cpus from entering panic(). Otherwise,
> + * resetting it so that other cpus can enter panic()/crash_kexec().
> + */
> + if (old_cpu == this_cpu)
> + atomic_set(&panicking_cpu, -1);
This do the opposite what the comment says, wouldn't it? You should
check old_cpu == -1. Also atomic_set doesn't imply memory barriers which
might be a problem.
--
Michal Hocko
SUSE Labs
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[V2 PATCH 2/3] kexec: Fix race between panic() and crash_kexec() called directly
Currently, panic() and crash_kexec() can be called at the same time.
For example (x86 case):
CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
nmi_shootdown_cpus() // stop other cpus
CPU 1:
panic()
crash_kexec()
mutex_trylock() // failed to acquire
smp_send_stop() // stop other cpus
infinite loop
If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump
fails.
In another case:
CPU 0:
oops_end()
crash_kexec()
mutex_trylock() // acquired
io_check_error()
panic()
crash_kexec()
mutex_trylock() // failed to acquire
infinite loop
Clearly, this is an undesirable result.
To fix this problem, this patch changes crash_kexec() to exclude
others by using atomic_t panicking_cpu.
V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock
to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()
Signed-off-by: Hidehiro Kawai
Cc: Eric Biederman
Cc: Vivek Goyal
Cc: Andrew Morton
---
kernel/kexec.c | 20
1 file changed, 20 insertions(+)
diff --git a/kernel/kexec.c b/kernel/kexec.c
index a785c10..ca40a19 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void)
void crash_kexec(struct pt_regs *regs)
{
+ int old_cpu, this_cpu;
+
+ /*
+* `old_cpu == -1' means we are the first comer and crash_kexec()
+* was called without entering panic().
+* `old_cpu == this_cpu' means crash_kexec() was called from panic().
+*/
+ this_cpu = raw_smp_processor_id();
+ old_cpu = atomic_cmpxchg(&panicking_cpu, -1, this_cpu);
+ if (old_cpu != -1 && old_cpu != this_cpu)
+ return;
+
/* Take the kexec_mutex here to prevent sys_kexec_load
* running on one cpu from replacing the crash kernel
* we are using after a panic on a different cpu.
@@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs)
}
mutex_unlock(&kexec_mutex);
}
+
+ /*
+* If we came here from panic(), we have to keep panicking_cpu
+* to prevent other cpus from entering panic(). Otherwise,
+* resetting it so that other cpus can enter panic()/crash_kexec().
+*/
+ if (old_cpu == this_cpu)
+ atomic_set(&panicking_cpu, -1);
}
size_t crash_get_memory_size(void)
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