@@ -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.
@@ -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
@@ -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
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> --- v2: Also update the failure code table. Documentation/powerpc/transactional_memory.txt | 32 ++++++++++++------------ arch/powerpc/include/uapi/asm/tm.h | 2 +- arch/powerpc/kernel/entry_64.S | 19 ++++++++++++++ 3 files changed, 36 insertions(+), 17 deletions(-)