Re: [PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

[Valentin Schneider <vschneid@redhat.com>]

On 29/06/22 13:55, Petr Mladek wrote:
> On Tue 2022-06-28 18:33:08, Valentin Schneider wrote:
>>
>> 8c5a1cf0ad3a ("kexec: use a mutex for locking rather than xchg()") was
>> straightforward enough because it turned
>>
>>         if (xchg(&lock, 1))
>>                 return -EBUSY;
>>
>> into
>>
>>         if (!mutex_trylock(&lock))
>>                 return -EBUSY;
>>
>> Now, most of the kexec_mutex uses are trylocks, except for:
>> - crash_get_memory_size()
>> - crash_shrink_memory()
>>
>> I really don't want to go down the route of turning those into cmpxchg
>> try-loops, would it be acceptable to make those use trylocks (i.e. return
>> -EBUSY if the cmpxchg fails)?
>
> IMHO, -EBUSY is acceptable for both crash_get_memory_size()
> and crash_shrink_memory(). They are used in the sysfs interface.
>
>> Otherwise, we keep the mutexes for functions like those which go nowhere
>> near an NMI.
>
> If we go this way then I would hide the locking into some wrappers,
> like crash_kexec_trylock()/unlock() that would do both mutex
> and xchg. The xchg part might be hidden in a separate wrapper
> __crash_kexec_trylock()/unlock() or
> crash_kexec_atomic_trylock()/unlock().
>

Makes sense, thanks. I've started playing with the trylock/-EBUSY approach,
I'll toss it out if I don't end up hating it.

> Best Regards,
> Petr


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