Bit-fields allow for a simple way to break portions of a struct into portions that only use up a certain specified number of bits. If multiple threads are accessing or making modifications to different bit-fields, a race condition may be present because the architecture may not be able to modify only the bits to which the currently being modified member may refer. Therefore, a mutex protecting all bit-fields at the same time must be used.
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Although this appears to be harmless, it is possible (and likely) that the architecture that this is running on has aligned flag1 and flag2 on the same byte. If both assignments occur on a thread scheduling interleaving which ends with the both stores occurring after one another, it is possible that only one of the flags will be set as intended and the other flag will equal its previous value due to the fact that both of the bit-fields fell on the same byte, which was the smallest unit the processor could work on.
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