This week at LWN: Atomic context and kernel API design
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An API should refrain from making promises that it cannot keep. A recent episode involving the kernel's in_atomic() macro demonstrates how things can go wrong when a function does not really do what it appears to do. It is also a good excuse to look at an under-documented (but fundamental) aspect of kernel code design. Kernel code generally runs in one of two fundamental contexts. Process context reigns when the kernel is running directly on behalf of a (usually) user-space process; the code which implements system calls is one example. When the kernel is running in process context, it is allowed to go to sleep if necessary. But when the kernel is running in atomic context, things like sleeping are not allowed. Code which handles hardware and software interrupts is one obvious example of atomic context.
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