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Fix race in futex_sleep

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Former-commit-id: 73300c57005a49bafbfc44db9c40ba7d1d4eedce
This commit is contained in:
John Sully 2020-05-31 16:02:57 -04:00
parent 4a110615f3
commit 891977d9b7
2 changed files with 13 additions and 5 deletions
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16
src/fastlock.cpp
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@ -293,12 +293,21 @@ uint64_t fastlock_getlongwaitcount()
return rval;
}
extern "C" void fastlock_sleep(fastlock *lock, pid_t pid, unsigned wake, unsigned mask)
extern "C" void fastlock_sleep(fastlock *lock, pid_t pid, unsigned wake, unsigned myticket)
{
#ifdef __linux__
g_dlock.registerwait(lock, pid);
unsigned mask = (1U << (myticket % 32));
__atomic_fetch_or(&lock->futex, mask, __ATOMIC_ACQUIRE);
futex(&lock->m_ticket.u, FUTEX_WAIT_BITSET_PRIVATE, wake, nullptr, mask);
// double check the lock wasn't release between the last check and us setting the futex mask
uint32_t u;
__atomic_load(&lock->m_ticket.u, &u, __ATOMIC_ACQUIRE);
if ((u & 0xffff) != myticket)
{
futex(&lock->m_ticket.u, FUTEX_WAIT_BITSET_PRIVATE, wake, nullptr, mask);
}
__atomic_fetch_and(&lock->futex, ~mask, __ATOMIC_RELEASE);
g_dlock.clearwait(lock, pid);
#endif
@ -332,7 +341,6 @@ extern "C" void fastlock_lock(struct fastlock *lock)
int tid = gettid();
unsigned myticket = __atomic_fetch_add(&lock->m_ticket.m_avail, 1, __ATOMIC_RELEASE);
unsigned mask = (1U << (myticket % 32));
unsigned cloops = 0;
ticket ticketT;
unsigned loopLimit = g_fHighCpuPressure ? 0x10000 : 0x100000;
@ -350,7 +358,7 @@ extern "C" void fastlock_lock(struct fastlock *lock)
#endif
if ((++cloops % loopLimit) == 0)
{
fastlock_sleep(lock, tid, ticketT.u, mask);
fastlock_sleep(lock, tid, ticketT.u, myticket);
}
}

2
src/fastlock_x64.asm
View File

@ -69,7 +69,7 @@ fastlock_lock:
# rdi ARG1 futex (already in rdi)
# rsi ARG2 tid (already in esi)
# rdx ARG3 ticketT.u (already in edx)
bts ecx, eax # rcx ARG4 mask
mov ecx, eax # rcx ARG4 myticket
call fastlock_sleep
# cleanup and continue
pop rax