configure: Check for membarrier support

[clementguidi]

This is the third PR in a series of patches intended to bring runtime dynamic tracing on x86_64 to uftrace.

1. dynamic: Refactor logic to perform multiple updates #1702
2. dynamic: Refactor patch pattern logic #1703
3. configure: Check for membarrier support #1704 🠈
4. utils: Add helpers for runtime dynamic patching #1705
5. dynamic: x86_64: refactor patch code #1745
6. dynamic: x86_64: Support runtime dynamic patching #1746
7. dynamic: x86_64: Optimize runtime dynamic patching #1747
8. dynamic: x86 64: Support runtime dynamic unpatching #1748
9. agent: Trigger runtime dynamic instrumentation #1749
10. dynamic: x86_64: Fall back to original patching strategy when target is not running #1750
11. dynamic: Display enhanced dynamic stats #1751

Runtime dynamic patching requires cache synchronization. This is best achieved by using the MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE memory barrier. However it was introduced in Linux 4.16. We set a flag
indicating whether this membarrier can be used or if libmcount has to rely on other mechanisms (e.g. signals).

We introduce a configuration option --without-membarrier to disable the use of MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE even if it is available.

Related: #1698

[namhyung]

By using _PRIVATE_EXPEDITED_SYNC_CORE, can we delete the old copy right after the syscall?

[clementguidi]

Sorry, what do you mean by "the old copy"?

Here is an extract from the membarrier manual, for future reference:

MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE (since Linux 4.16)

In addition to providing the memory ordering guarantees described in MEMBARRIER_CMD_PRIVATE_EXPEDITED, upon return from system call the calling thread has a guarantee that all its running thread siblings have executed a core serializing instruction. 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.

MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE (since Linux 4.16)

Register the process's intent to use MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE.

[namhyung]

Sorry for the late reply. I think you want to use membarrier to update global state like in a RCU fashion. Something like below:

new_data = copy_data(shared_data);
add_some_data(new_data);

old_data = shared_data;
shared_data = new_data;  // WRITE_ONCE ?

membarrier();

free(old_data);

[azharivs]

That is somewhat true. We want to use membarriers to stop all running threads before entering patching/unpatching code as will be done in this other commit:
f07ab57

Note that this particular PR merely checks for availability of membarrier system call. The actual usage is left for PR #1747

[namhyung]

I know, I was just curious how it's gonna be used.