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For parallel I/O with multiple concurrent threads servicing the same device (or devices, if several share a domain), serialising page table updates becomes a massive bottleneck. On reflection, though, we don't strictly need to do that - for valid IOMMU API usage, there are in fact only two races that we need to guard against: multiple map requests for different blocks within the same region, when the intermediate-level table for that region does not yet exist; and multiple unmaps of different parts of the same block entry. Both of those are fairly easily solved by using a cmpxchg to install the new table, such that if we then find that someone else's table got there first, we can simply free ours and continue. Make the requisite changes such that we can withstand being called without the caller maintaining a lock. In theory, this opens up a few corners in which wildly misbehaving callers making nonsensical overlapping requests might lead to crashes instead of just unpredictable results, but correct code really does not deserve to pay a significant performance cost for the sake of masking bugs in theoretical broken code. Signed-off-by: Robin Murphy <robin.murphy@arm.com> Signed-off-by: Will Deacon <will.deacon@arm.com> |
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| README | ||
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.