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powerpc/tm: Abort syscalls in active transactions

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This patch changes the syscall handler to doom (tabort) active
transactions when a syscall is made and return immediately without
performing the syscall.

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 because
syscalls will still be performed after tsuspend. 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 system call increases by about 0.5%.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Acked-By: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Sam bobroff 2015-04-10 14:16:47 +10:00 committed by Michael Ellerman
parent 771e569e82
commit feba40362b
3 changed files with 36 additions and 17 deletions
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32
Documentation/powerpc/transactional_memory.txt
View file

@ -74,22 +74,23 @@ Causes of transaction aborts
Syscalls
========
Performing syscalls from within transaction is not recommended, and can lead
to unpredictable results.
Syscalls made from within an active transaction will not be performed and the
transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL
| TM_CAUSE_PERSISTENT.
Syscalls do not by design abort transactions, but beware: The kernel code will
not be running in transactional state. The effect of syscalls will always
remain visible, but depending on the call they may abort your transaction as a
side-effect, read soon-to-be-aborted transactional data that should not remain
invisible, etc. If you constantly retry a transaction that constantly aborts
itself by calling a syscall, you'll have a livelock & make no progress.
Syscalls made from within a suspended transaction are performed as normal and
the transaction is not explicitly doomed by the kernel. However, what the
kernel does to perform the syscall may result in the transaction being doomed
by the hardware. The syscall is performed in suspended mode so any side
effects will be persistent, independent of transaction success or failure. No
guarantees are provided by the kernel about which syscalls will affect
transaction success.
Simple syscalls (e.g. sigprocmask()) "could" be OK. Even things like write()
from, say, printf() should be OK as long as the kernel does not access any
memory that was accessed transactionally.
Consider any syscalls that happen to work as debug-only -- not recommended for
production use. Best to queue them up till after the transaction is over.
Care must be taken when relying on syscalls to abort during active transactions
if the calls are made via a library. Libraries may cache values (which may
give the appearance of success) or perform operations that cause transaction
failure before entering the kernel (which may produce different failure codes).
Examples are glibc's getpid() and lazy symbol resolution.
Signals
@ -176,8 +177,7 @@ kernel aborted a transaction:
TM_CAUSE_RESCHED Thread was rescheduled.
TM_CAUSE_TLBI Software TLB invalide.
TM_CAUSE_FAC_UNAV FP/VEC/VSX unavailable trap.
TM_CAUSE_SYSCALL Currently unused; future syscalls that must abort
transactions for consistency will use this.
TM_CAUSE_SYSCALL Syscall from active transaction.
TM_CAUSE_SIGNAL Signal delivered.
TM_CAUSE_MISC Currently unused.
TM_CAUSE_ALIGNMENT Alignment fault.

2
arch/powerpc/include/uapi/asm/tm.h
View file

@ -11,7 +11,7 @@
#define TM_CAUSE_RESCHED 0xde
#define TM_CAUSE_TLBI 0xdc
#define TM_CAUSE_FAC_UNAV 0xda
#define TM_CAUSE_SYSCALL 0xd8 /* future use */
#define TM_CAUSE_SYSCALL 0xd8
#define TM_CAUSE_MISC 0xd6 /* future use */
#define TM_CAUSE_SIGNAL 0xd4
#define TM_CAUSE_ALIGNMENT 0xd2

19
arch/powerpc/kernel/entry_64.S
View file

@ -34,6 +34,7 @@
#include <asm/ftrace.h>
#include <asm/hw_irq.h>
#include <asm/context_tracking.h>
#include <asm/tm.h>
/*
* System calls.
@ -145,6 +146,24 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
andi. r11,r10,_TIF_SYSCALL_DOTRACE
bne syscall_dotrace
.Lsyscall_dotrace_cont:
#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
BEGIN_FTR_SECTION
b 1f
END_FTR_SECTION_IFCLR(CPU_FTR_TM)
extrdi. r11, r12, 1, (63-MSR_TS_T_LG) /* transaction active? */
beq+ 1f
/* Doom the transaction and don't perform the syscall: */
mfmsr r11
li r12, 1
rldimi r11, r12, MSR_TM_LG, 63-MSR_TM_LG
mtmsrd r11, 0
li r11, (TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT)
TABORT(R11)
b .Lsyscall_exit
1:
#endif
cmpldi 0,r0,NR_syscalls
bge- syscall_enosys