mmc: correct semantics of the mmc_host_remove

[Alex Dubov <oakad@yahoo.com>]

Greetings.

It appears to me that under certain circumstances mmc layer will issue requests to the host after
mmc_host_remove returns. This happens, for example, in tifm_sd driver because mmc_host may be
removed mid-transfer, as the socket shall be freed for possible reuse by different media type.
Currently, the only solution is to sleep a little somewhere after mmc_remove_host but before
mmc_free_host. I think the correct way to handle the situation is to ensure that mmc_host is never
accessed by the mmc layer after mmc_remove_host returns.


I think a easy way to handle this is to modify __mmc_claim_host to fail if the mmc_host is marked
for removal (this implies that its return value should be checked on use, which is not currently
the case everywhere). This way, mmc_host_remove can claim host, mark it as "dead" and then return
safely knowing that nobody will send any more requests to the host. 

Some questions:
1. Will this suffice for the task? 
2. Are there any reasons not to do this?
3. Is it possible to replace the fancy locking loop in the __mmc_claim_host with mutex based
locking (mutex does the same thing, isn't it)?



 
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