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

[Valentin Schneider]

On 27/06/22 13:42, Valentin Schneider wrote:
> On 25/06/22 12:04, Eric W. Biederman wrote:
>> At this point I recommend going back to being ``unconventional'' with
>> the kexec locking and effectively reverting commit 8c5a1cf0ad3a ("kexec:
>> use a mutex for locking rather than xchg()").
>>
>> That would also mean that we don't have to worry about the lockdep code
>> doing something weird in the future and breaking kexec.
>>
>> Your change starting to is atomic_cmpxchng is most halfway to a revert
>> of commit 8c5a1cf0ad3a ("kexec: use a mutex for locking rather than
>> xchg()").  So we might as well go the whole way and just document that
>> the kexec on panic code can not use conventional kernel locking
>> primitives and has to dig deep and build it's own.  At which point it
>> makes no sense for the rest of the kexec code to use anything different.
>>
>
> Hm, I'm a bit torn about that one, ideally I'd prefer to keep "homegrown"
> locking primitives to just where they are needed (loading & kexec'ing), but
> I'm also not immensely fond of the "hybrid" mutex+cmpxchg approach.
>

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)?

Otherwise, we keep the mutexes for functions like those which go nowhere
near an NMI.