Re: [PATCH v3 6/7] util/oslib-posix: Support concurrent os_mem_prealloc() invocation

[David Hildenbrand <david@redhat.com>]

On 28.09.21 18:56, Daniel P. Berrangé wrote:
> On Mon, Aug 16, 2021 at 11:47:38AM +0200, David Hildenbrand wrote:
>> Add a mutex to protect the SIGBUS case, as we cannot mess concurrently
>> with the sigbus handler and we have to manage the global variable
>> sigbus_memset_context. The MADV_POPULATE_WRITE path can run
>> concurrently.
>>
>> Note that page_mutex and page_cond are shared between concurrent
>> invocations, which shouldn't be a problem.
>>
>> This is a preparation for future virtio-mem prealloc code, which will call
>> os_mem_prealloc() asynchronously from an iothread when handling guest
>> requests.
>>
>> Add a comment that messing with the SIGBUS handler is frowned upon and
>> can result in problems we fortunately haven't seen so far. Note that
>> forwarding signals to the already installed SIGBUS handler isn't clean
>> either, as that one might modify the SIGBUS handler again.
> 
> Even with the mutex, messing with SIGBUS post-startup still isn't safe
> as we're clashing with SIGBUS usage in softmmu/cpus.c

You raise a valid point, though: this node is stale (there is no such 
comment) and shall be removed :)

Thanks!

-- 
Thanks,

David / dhildenb