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![Sam bobroff](/assets/img/avatar_default.png)
This patch changes the syscall handler to doom (tabort) active transactions when a syscall is made and return very early without performing the syscall and keeping side effects to a minimum (no CPU accounting or system call tracing is performed). Also included is a new HWCAP2 bit, PPC_FEATURE2_HTM_NOSC, to indicate this behaviour to userspace. Currently, the system call instruction automatically suspends an active transaction which causes side effects to persist when an active transaction fails. This does change the kernel's behaviour, but in a way that was documented as unsupported. It doesn't reduce functionality as syscalls will still be performed after tsuspend; it just requires that the transaction be explicitly suspended. It also provides a consistent interface and makes the behaviour of user code substantially the same across powerpc and platforms that do not support suspended transactions (e.g. x86 and s390). Performance measurements using http://ozlabs.org/~anton/junkcode/null_syscall.c indicate the cost of a normal (non-aborted) system call increases by about 0.25%. Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
198 lines
7.9 KiB
Text
198 lines
7.9 KiB
Text
Transactional Memory support
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============================
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POWER kernel support for this feature is currently limited to supporting
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its use by user programs. It is not currently used by the kernel itself.
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This file aims to sum up how it is supported by Linux and what behaviour you
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can expect from your user programs.
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Basic overview
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==============
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Hardware Transactional Memory is supported on POWER8 processors, and is a
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feature that enables a different form of atomic memory access. Several new
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instructions are presented to delimit transactions; transactions are
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guaranteed to either complete atomically or roll back and undo any partial
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changes.
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A simple transaction looks like this:
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begin_move_money:
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tbegin
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beq abort_handler
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ld r4, SAVINGS_ACCT(r3)
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ld r5, CURRENT_ACCT(r3)
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subi r5, r5, 1
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addi r4, r4, 1
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std r4, SAVINGS_ACCT(r3)
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std r5, CURRENT_ACCT(r3)
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tend
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b continue
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abort_handler:
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... test for odd failures ...
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/* Retry the transaction if it failed because it conflicted with
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* someone else: */
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b begin_move_money
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The 'tbegin' instruction denotes the start point, and 'tend' the end point.
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Between these points the processor is in 'Transactional' state; any memory
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references will complete in one go if there are no conflicts with other
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transactional or non-transactional accesses within the system. In this
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example, the transaction completes as though it were normal straight-line code
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IF no other processor has touched SAVINGS_ACCT(r3) or CURRENT_ACCT(r3); an
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atomic move of money from the current account to the savings account has been
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performed. Even though the normal ld/std instructions are used (note no
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lwarx/stwcx), either *both* SAVINGS_ACCT(r3) and CURRENT_ACCT(r3) will be
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updated, or neither will be updated.
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If, in the meantime, there is a conflict with the locations accessed by the
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transaction, the transaction will be aborted by the CPU. Register and memory
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state will roll back to that at the 'tbegin', and control will continue from
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'tbegin+4'. The branch to abort_handler will be taken this second time; the
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abort handler can check the cause of the failure, and retry.
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Checkpointed registers include all GPRs, FPRs, VRs/VSRs, LR, CCR/CR, CTR, FPCSR
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and a few other status/flag regs; see the ISA for details.
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Causes of transaction aborts
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============================
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- Conflicts with cache lines used by other processors
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- Signals
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- Context switches
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- See the ISA for full documentation of everything that will abort transactions.
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Syscalls
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========
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Syscalls made from within an active transaction will not be performed and the
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transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL
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| TM_CAUSE_PERSISTENT.
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Syscalls made from within a suspended transaction are performed as normal and
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the transaction is not explicitly doomed by the kernel. However, what the
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kernel does to perform the syscall may result in the transaction being doomed
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by the hardware. The syscall is performed in suspended mode so any side
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effects will be persistent, independent of transaction success or failure. No
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guarantees are provided by the kernel about which syscalls will affect
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transaction success.
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Care must be taken when relying on syscalls to abort during active transactions
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if the calls are made via a library. Libraries may cache values (which may
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give the appearance of success) or perform operations that cause transaction
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failure before entering the kernel (which may produce different failure codes).
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Examples are glibc's getpid() and lazy symbol resolution.
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Signals
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=======
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Delivery of signals (both sync and async) during transactions provides a second
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thread state (ucontext/mcontext) to represent the second transactional register
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state. Signal delivery 'treclaim's to capture both register states, so signals
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abort transactions. The usual ucontext_t passed to the signal handler
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represents the checkpointed/original register state; the signal appears to have
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arisen at 'tbegin+4'.
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If the sighandler ucontext has uc_link set, a second ucontext has been
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delivered. For future compatibility the MSR.TS field should be checked to
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determine the transactional state -- if so, the second ucontext in uc->uc_link
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represents the active transactional registers at the point of the signal.
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For 64-bit processes, uc->uc_mcontext.regs->msr is a full 64-bit MSR and its TS
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field shows the transactional mode.
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For 32-bit processes, the mcontext's MSR register is only 32 bits; the top 32
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bits are stored in the MSR of the second ucontext, i.e. in
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uc->uc_link->uc_mcontext.regs->msr. The top word contains the transactional
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state TS.
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However, basic signal handlers don't need to be aware of transactions
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and simply returning from the handler will deal with things correctly:
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Transaction-aware signal handlers can read the transactional register state
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from the second ucontext. This will be necessary for crash handlers to
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determine, for example, the address of the instruction causing the SIGSEGV.
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Example signal handler:
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void crash_handler(int sig, siginfo_t *si, void *uc)
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{
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ucontext_t *ucp = uc;
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ucontext_t *transactional_ucp = ucp->uc_link;
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if (ucp_link) {
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u64 msr = ucp->uc_mcontext.regs->msr;
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/* May have transactional ucontext! */
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#ifndef __powerpc64__
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msr |= ((u64)transactional_ucp->uc_mcontext.regs->msr) << 32;
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#endif
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if (MSR_TM_ACTIVE(msr)) {
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/* Yes, we crashed during a transaction. Oops. */
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fprintf(stderr, "Transaction to be restarted at 0x%llx, but "
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"crashy instruction was at 0x%llx\n",
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ucp->uc_mcontext.regs->nip,
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transactional_ucp->uc_mcontext.regs->nip);
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}
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}
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fix_the_problem(ucp->dar);
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}
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When in an active transaction that takes a signal, we need to be careful with
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the stack. It's possible that the stack has moved back up after the tbegin.
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The obvious case here is when the tbegin is called inside a function that
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returns before a tend. In this case, the stack is part of the checkpointed
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transactional memory state. If we write over this non transactionally or in
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suspend, we are in trouble because if we get a tm abort, the program counter and
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stack pointer will be back at the tbegin but our in memory stack won't be valid
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anymore.
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To avoid this, when taking a signal in an active transaction, we need to use
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the stack pointer from the checkpointed state, rather than the speculated
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state. This ensures that the signal context (written tm suspended) will be
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written below the stack required for the rollback. The transaction is aborted
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because of the treclaim, so any memory written between the tbegin and the
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signal will be rolled back anyway.
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For signals taken in non-TM or suspended mode, we use the
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normal/non-checkpointed stack pointer.
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Failure cause codes used by kernel
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==================================
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These are defined in <asm/reg.h>, and distinguish different reasons why the
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kernel aborted a transaction:
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TM_CAUSE_RESCHED Thread was rescheduled.
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TM_CAUSE_TLBI Software TLB invalid.
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TM_CAUSE_FAC_UNAV FP/VEC/VSX unavailable trap.
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TM_CAUSE_SYSCALL Syscall from active transaction.
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TM_CAUSE_SIGNAL Signal delivered.
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TM_CAUSE_MISC Currently unused.
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TM_CAUSE_ALIGNMENT Alignment fault.
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TM_CAUSE_EMULATE Emulation that touched memory.
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These can be checked by the user program's abort handler as TEXASR[0:7]. If
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bit 7 is set, it indicates that the error is consider persistent. For example
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a TM_CAUSE_ALIGNMENT will be persistent while a TM_CAUSE_RESCHED will not.
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GDB
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===
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GDB and ptrace are not currently TM-aware. If one stops during a transaction,
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it looks like the transaction has just started (the checkpointed state is
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presented). The transaction cannot then be continued and will take the failure
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handler route. Furthermore, the transactional 2nd register state will be
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inaccessible. GDB can currently be used on programs using TM, but not sensibly
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in parts within transactions.
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