----- On Dec 28, 2020, at 3:24 PM, Russell King, ARM Linux linux@armlinux.org.uk wrote:
On Mon, Dec 28, 2020 at 11:44:33AM -0800, Andy Lutomirski wrote:
On Mon, Dec 28, 2020 at 11:09 AM Russell King - ARM Linux admin linux@armlinux.org.uk wrote:
On Mon, Dec 28, 2020 at 07:29:34PM +0100, Jann Horn wrote:
After chatting with rmk about this (but without claiming that any of this is his opinion), based on the manpage, I think membarrier() currently doesn't really claim to be synchronizing caches? It just serializes cores. So arguably if userspace wants to use membarrier() to synchronize code changes, userspace should first do the code change, then flush icache as appropriate for the architecture, and then do the membarrier() to ensure that the old code is unused?
For 32-bit arm, rmk pointed out that that would be the cacheflush() syscall. That might cause you to end up with two IPIs instead of one in total, but we probably don't care _that_ much about extra IPIs on 32-bit arm?
For arm64, I believe userspace can flush icache across the entire system with some instructions from userspace - "DC CVAU" followed by "DSB ISH", or something like that, I think? (See e.g. compat_arm_syscall(), the arm64 compat code that implements the 32-bit arm cacheflush() syscall.)
Note that the ARM cacheflush syscall calls flush_icache_user_range() over the range of addresses that userspace has passed - it's intention since day one is to support cases where userspace wants to change executable code.
It will issue the appropriate write-backs to the data cache (DCCMVAU), the invalidates to the instruction cache (ICIMVAU), invalidate the branch target buffer (BPIALLIS or BPIALL as appropriate), and issue the appropriate barriers (DSB ISHST, ISB).
Note that neither flush_icache_user_range() nor flush_icache_range() result in IPIs; cache operations are broadcast across all CPUs (which is one of the minimums we require for SMP systems.)
Now, that all said, I think the question that has to be asked is...
What is the basic purpose of membarrier?Is the purpose of it to provide memory barriers, or is it to provide memory coherence?
If it's the former and not the latter, then cache flushes are out of scope, and expecting memory written to be visible to the instruction stream is totally out of scope of the membarrier interface, whether or not the writes happen on the same or a different CPU to the one executing the rewritten code.
The documentation in the kernel does not seem to describe what it's supposed to be doing - the only thing I could find is this: Documentation/features/sched/membarrier-sync-core/arch-support.txt which describes it as "arch supports core serializing membarrier" whatever that means.
Seems to be the standard and usual case of utterly poor to non-existent documentation within the kernel tree, or even a pointer to where any useful documentation can be found.
Reading the membarrier(2) man page, I find nothing in there that talks about any kind of cache coherency for self-modifying code - it only seems to be about _barriers_ and nothing more, and barriers alone do precisely nothing to save you from non-coherent Harvard caches.
So, either Andy has a misunderstanding, or the man page is wrong, or my rudimentary understanding of what membarrier is supposed to be doing is wrong...
Look at the latest man page:
https://man7.org/linux/man-pages/man2/membarrier.2.html
for MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE. The result may not be all that enlightening.
MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE (since Linux 4.16) In addition to providing the memory ordering guarantees de■ scribed in MEMBARRIER_CMD_PRIVATE_EXPEDITED, upon return from system call the calling thread has a guarantee that all its run■ ning thread siblings have executed a core serializing instruc■ tion. This guarantee is provided only for threads in the same process as the calling thread. The "expedited" commands complete faster than the non-expedited ones, they never block, but have the downside of causing extra overhead. A process must register its intent to use the private expedited sync core command prior to using it.This just says that the siblings have executed a serialising instruction, in other words a barrier. It makes no claims concerning cache coherency - and without some form of cache maintenance, there can be no expectation that the I and D streams to be coherent with each other.
Right, membarrier is not doing anything wrt I/D caches. On architectures without coherent caches, users should use other system calls or instructions provided by the architecture to synchronize the appropriate address ranges.
This description is also weird in another respect. "guarantee that all its running thread siblings have executed a core serializing instruction" ... "The expedited commands ... never block".
So, the core executing this call is not allowed to block, but the other part indicates that the other CPUs _have_ executed a serialising instruction before this call returns... one wonders how that happens without blocking. Maybe the CPU spins waiting for completion instead?
Membarrier expedited sync-core issues IPIs to all CPUs running sibling threads. AFAIR the IPI mechanism uses the "csd lock" which is basically busy waiting. So it does not "block", it busy-waits.
For completeness of the explanation, other (non-running) threads acting on the same mm will eventually issue the context synchronizing instruction before returning to user-space whenever they are scheduled back.
Thanks,
Mathieu
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