thread safe fat

Hi,
I'm using in a project of mine a free FAT filesystem to manage an sd
card.
Im' using an ATmega64 so I have not too much RAM. I also need to use
an OS to manage different tasks and can have File system operation
requests from different tasks. Is there any intrinsic free thread safe
implementation of FAT or have to manage that on my own using
semaphores or other synchronization tools?
The problem is that if task1 execs a fileopen, filewrite, fileclose
sequence and task2 exec a fopen in the middle of the previous sequence
is quite likely that tha internal data buffers of the FAT module
become corrupted.

Thanks

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