Arm64 atomic inc fix #10223
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Arm64 atomic inc fix #10223
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BPF programs detect recursion by doing atomic inc/dec on a per-cpu active counter from the trampoline. Create two helpers for operations on this active counter, this makes it easy to make changes later. Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
BPF programs detect recursion by a per-cpu active flag in struct
bpf_prog. This flag is set/unset through the trampoline using atomic
operations.
Some arm64 platforms have slow per-CPU atomic operations, for example,
the Neoverse V2. This commit therefore changes the recursion detection
mechanism to not use atomics at all.
Change active to a per-cpu array of four u8 values, one for each context
and use non-atomic increment/decrement on the value for the running
context. After the increment, check for inter-context / same-context
recursion by looking at the whole array as a u32 and checking if any
other array element other than the expected element has changed.
So, initally the array is {0,0,0,0} for {NMI, hard-irq, soft-irq, normal}
Let's say the bpf program is triggered on normal context so after the
increment this array looks like: {0,0,0,1} now let's say NMI is received
on this CPU and the bpf program is again triggered in NMI context, so
the increment happens again the array now looks like: {1,0,0,1}. Now
when the NMI context checks the u32 for the value 0x000001, it will find
0x01000001 and recursion will be detected. Similarly, recursion from the
same context can be detected.
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
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