[Pcsclite-muscle] CCID handling of multiple devices

[Ludovic Rousseau]

2016-04-20 18:57 GMT+02:00 Maksim Ivanov <emaxx at google.com>:

> Hello,
>

Hello,


>
> Looking at the CCID source code, it seems that there may be some
> problems with working with multiple readers simultaneously.
>
> The CCID reader polling code (or, to be precise, its libusb-based
> version) is based on the libusb_handle_events function:
>
> https://alioth.debian.org/plugins/scmgit/cgi-bin/gitweb.cgi?p=pcsclite/CCID.git;a=blob;f=src/ccid_usb.c;h=6a097eadb821ebed39eefac6c4b39b3f8f135001#l1252
>
> The problem with this function is that, as it's documented in libusb
> docs, it's prone to race conditions:
> http://libusb.sourceforge.net/api-1.0/mtasync.html#Using
>
> Seems that the races are pretty likely to occur when there are two or
> more readers operated simultaneously. For each of them, a thread with
> a polling transfer is created by the CCID driver; each thread is
> blocked on libusb_handle_events most of its time. The first type of
> race may happen between checking the "completed" variable and calling
> the libusb_handle_events function: another thread with the running
> libusb_handle_events function may finish this transfer during this
> period of time. The second type of race roots from the fact that there
> is no guarantee which thread's libusb_handle_event will catch the
> events, so it may be that one polling thread misses the transfer
> completion it's waiting for.
>
> Even though the races, when they occur, result in just 60 seconds
> delay, it would be great if they would be omitted completely.
> The fix shouldn't be difficult, based on the handy function
> libusb_handle_events_completed, which appears to be solving exactly
> this problem:
>
> http://libusb.sourceforge.net/api-1.0/group__poll.html#ga0bc99f39e4cf5ad393cd5936c36037d1
>


Fixed in
https://github.com/LudovicRousseau/CCID/commit/2c8ffab9962484653b6445a6c85a6c690a5948b3

Thanks

-- 
 Dr. Ludovic Rousseau
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