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The current wrap implementation has a race issue: it is called outside of the ctx_alloc_lock, and also does not wait for all CPUs to complete the wrap. This means that a thread can get a new context with a new version and another thread might still be running with the same context. The problem is especially severe on CPUs with shared TLBs, like sun4v. I used the following test to very quickly reproduce the problem: - start over 8K processes (must be more than context IDs) - write and read values at a memory location in every process. Very quickly memory corruptions start happening, and what we read back does not equal what we wrote. Several approaches were explored before settling on this one: Approach 1: Move smp_new_mmu_context_version() inside ctx_alloc_lock, and wait for every process to complete the wrap. (Note: every CPU must WAIT before leaving smp_new_mmu_context_version_client() until every one arrives). This approach ends up with deadlocks, as some threads own locks which other threads are waiting for, and they never receive softint until these threads exit smp_new_mmu_context_version_client(). Since we do not allow the exit, deadlock happens. Approach 2: Handle wrap right during mondo interrupt. Use etrap/rtrap to enter into into C code, and issue new versions to every CPU. This approach adds some overhead to runtime: in switch_mm() we must add some checks to make sure that versions have not changed due to wrap while we were loading the new secondary context. (could be protected by PSTATE_IE but that degrades performance as on M7 and older CPUs as it takes 50 cycles for each access). Also, we still need a global per-cpu array of MMs to know where we need to load new contexts, otherwise we can change context to a thread that is going way (if we received mondo between switch_mm() and switch_to() time). Finally, there are some issues with window registers in rtrap() when context IDs are changed during CPU mondo time. The approach in this patch is the simplest and has almost no impact on runtime. We use the array with mm's where last secondary contexts were loaded onto CPUs and bump their versions to the new generation without changing context IDs. If a new process comes in to get a context ID, it will go through get_new_mmu_context() because of version mismatch. But the running processes do not need to be interrupted. And wrap is quicker as we do not need to xcall and wait for everyone to receive and complete wrap. Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com> Reviewed-by: Bob Picco <bob.picco@oracle.com> Reviewed-by: Steven Sistare <steven.sistare@oracle.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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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.