Commit c17a113e authored by Yedidya Feldblum's avatar Yedidya Feldblum Committed by Facebook Github Bot 2

Make the mprotect variant of asymmetricHeavyBarrier work when mlock fails

Summary: [Folly] Make the `mprotect` variant of `asymmetricHeavyBarrier` work when `mlock` fails.

Reviewed By: djwatson

Differential Revision: D3585948

fbshipit-source-id: c3a46884434b7f9da9caa9cf203573f9e3ce7444
parent 7dd33144
......@@ -42,16 +42,18 @@ struct DummyPageCreator {
auto ptr = mmap(nullptr, 1, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
checkUnixError(reinterpret_cast<uintptr_t>(ptr), "mmap");
// Lock the memory so it can't get paged out. If it gets paged out, changing
// its protection won't accomplish anything.
// Optimistically try to lock the page so it stays resident. Could make
// the heavy barrier faster.
auto r = mlock(ptr, 1);
checkUnixError(r, "mlock");
if (r != 0) {
// Do nothing.
}
return ptr;
}
};
// Make sure dummy page is always initialized before shutdown
// Make sure dummy page is always initialized before shutdown.
DummyPageCreator dummyPageCreator;
void mprotectMembarrier() {
......@@ -63,9 +65,17 @@ void mprotectMembarrier() {
std::lock_guard<std::mutex> lg(*mprotectMutex);
int r = 0;
// We want to downgrade the page while it is resident. To do that, it must
// first be upgraded and forced to be resident.
r = mprotect(dummyPage, 1, PROT_READ | PROT_WRITE);
checkUnixError(r, "mprotect");
// Force the page to be resident. If it is already resident, almost no-op.
*static_cast<char*>(dummyPage) = 0;
// Downgrade the page. Forces a memory barrier in every core running any
// of the process's threads. On a sane platform.
r = mprotect(dummyPage, 1, PROT_READ);
checkUnixError(r, "mprotect");
}
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment