configure: Check for membarrier support #1704
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This was referenced May 11, 2023
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). Signed-off-by: Clément Guidi <cguidi@ciena.com>
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This was referenced Jul 7, 2023
namhyung
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Jul 10, 2023
| int main() | ||
| { | ||
| syscall(__NR_membarrier, MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE, 0, 0); | ||
| syscall(__NR_membarrier, MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE, 0, 0); |
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By using _PRIVATE_EXPEDITED_SYNC_CORE, can we delete the old copy right after the syscall?
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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.
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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);
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I know, I was just curious how it's gonna be used.
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This is the third PR in a series of patches intended to bring runtime dynamic tracing on x86_64 to uftrace.
Runtime dynamic patching requires cache synchronization. This is best achieved by using the
MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_COREmemory barrier. However it was introduced in Linux 4.16. We set a flagindicating whether this membarrier can be used or if libmcount has to rely on other mechanisms (e.g. signals).
We introduce a configuration option
--without-membarrierto disable the use ofMEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_COREeven if it is available.Related: #1698